i am wondering is it possible to put a gpu in a dell dimension 2400 there is no slot for a gpu but there is a place to put the slot if i solder a slot on there would it work? before you even get to that step, ask yourself is your soldering technique good enough to handle more than 100 solder spots, 1mm apart without cooking nearby components?
I know mine isn't
well maybe if its possible im going to try
Just remember that the factory NEVER solders these components by hand. Be prepared to toss your Mobo if it screws up.I admire your initiave....

I question your logic.              lolHow will the motherboard chipset deal with a new slot, which it was not designed for? You can fit a PCI display card anyhow.
If you look at the motherboard you will likely see other components missing in this region of the board!!! Which means you may be able to add a AGP connector, but the supporting circuits are open and missing paths to make it work properly.

I use to work in PCB Manufacturing and when we built a product that was for a specific build or CUSTOMER request that was different in that it was not the full system but instead a stripped down version of the full blown system it was never just lets not put a connector in, generally engineers removed all unnecessary components related to it as well as some systems even had different firmware so that the system would skip over a portion of the board/system that doesnt exist on that specific build.

Motherboards such as the one you have went through a wave soldering machine, where boards are held in place by titanium fingers that hold the edge of the board, and these fingers are connected to 2 chains that bring the boards through the wave solder machine at a speed of travel that is in control of the wave solder operator. These boards go through a fluxer that uses pressurized air to bubble the flux into a brush that blushes the bottom of the board with flux. Before the board with flux is brought to the solder pot it travels over heating elements to bring the board temperature up before it gets to the solder pot and this protects the PCB's from temperature shock but also the flux wicks up the legs of the tinned parts during this process. The boards then travel to the solder pot and a sump pumps up molten LEAD to a Z-baffle where the wave solder operator controls a knob that controls the flow of solder ( height of the solder exiting the Z-baffle ) which pours back into the HOT solder pot as liquid lead to MAINTAIN the liquid lead flow. The operator needs to be very careful not to have the height of the molten lead too high or it will pour over the top of the board and cost the company money in damaged PCB's but also not too low that parts of the board are not soldered. When the PCB's exited the solder pot they then where inspected and then washed in a large dishwasher for motherboards. Fortunately I got hired there shortly after they got rid of the freon bathing of boards to remove excess flux. The boards then go into a dryer that drys them so they dont corrode. Then they go to post-wave to add parts or Quality Control who look over boards for parts backwards, not seated properly, or wrong etc. Then the board goes for a burn in test if the company performs burn in testing such as we had a room that was 150F that the boards would run powered for 12 hours to stress the components then test the boards on a ATE or bench test them one by one to make sure all is well. Problem boards were sent to Service department for Electronics Technicians like myself could fix them and send them back out as good or toss them if they are beyond economical repair.

The wave solder machine makes for fast and proper soldering of PCB's as well as there are rarely issues with heatsinking causing inadequate soldering. Heatsinking is the problem where there is a large metal mass that is taking solder and the heat is drawn away on contact with the molten lead that the lead is no longer up to proper temperature to flow correctly into the solder joint. This usually causes legs of components to be soldered on the underside, but looking at the top of the board the leg(s) of the component do not have flow of solder up through the Barrel ( the hole that the component legs go through which is usually copper and by which it joins multiple layers of circuitry on boards to the component leg that is inserted and soldered into that location ) and this can cause problems in addition to the fact that it looks like a poor quality board to discover poor solder joints.

That all being said, the connector you want to install was never masked over and so the barrels likely are all plugged with lead. You would need to carefully remove the lead from the barrels without overheating the board and damaging it. This requires a solder sucker and skill in using one to not damage the board. Some people have used flux and solder wick as well if they dont have a solder sucker, but a solder sucker is the best method of lead removal from a barrel without damaging the PCB masking and barrels.

If components were intended to be added later a mask is placed over the connector location for example where latex is added over the board area on the underside and sometimes topside too if gold finger contacts are involved, however most manufacturers done away with latex poured over the area because it takes time to set, and have moved on to film tape like 3M 5413 Polyimide that we use to use lots of. When board is done with wave solder the tape peels off easily and you can add whatever component later, or now have gold fingers on the edge of a board that are nice gold color and not tinned with solder.

Knowing what I know, I wouldnt personally go the route of adding that connector. I'd just get a replacement motherboard that has the AGP connector. Here is one for sale cheap for $21.99 with free shipping at Newegg! https://www.newegg.com/Product/Product.aspx?Item=9SIA9AX65B1964&ignorebbr=1&nm_mc=KNC-GoogleMKP-PC&cm_mmc=KNC-GoogleMKP-PC-_-pla-_-Desktop+PC-_-9SIA9AX65B1964&gclid=EAIaIQobChMIi82zm86f3AIVU-DICh1_YAOjEAQYAiABEgL54PD_BwE&gclsrc=aw.ds

I worked at Allen-Bradley / Rockwell Automation from 1995 to 2001 and did just about every job in Production Department where needed including Wave Solder Operator when the main guy that ran it was out sick etc. Had 2 fires in the wave solder machine in the 6 years there because the flux was a mixture of Isopropyl Alcohol and Flux as a specific gravity had to be measured to have a flux that would be applied that wasnt too heavy or thin of flux, and the heating elements were hot red glowing elements like an electric oven and there was nothing protecting the red hot elements from a droplet of isopropyl/flux mixture dripping on it and flashing over which then catches the bottom of the boards on fire and the flame rushes back to the flux bubbler with brush. All that plus high volume airflow to VENT out the toxic fumes made the inside of the wave solder act like a flame thrower was  behind the glass windowed metal doors. The fire risk was removed when we moved on to newer flux that didnt use isopropyl alcohol however it was far more difficult to solder with because it loved to spider web solder the bottoms on the boards where a board came out with lead bridged between legs all over as well as solder balls that would spit out the tops of the barrels and made a mess for testing and reliability of boards until we got the settings just right to reduce solder balls and have a properly soldered product. For a short while we would take a board and tap it edgewise on a ESD mat on a table and it looked like 100 ball point pen balls fell from the board in how tiny the balls of lead were that were a risk for shorting component legs to other legs etc. When i left in 2001 we even moved away from lead solder to a lead-free solder and that in itself also had issues where lead solder works soooo much nicer, but they wanted to make boards that were lead-free.