does anyone KNOW how to add wifi to an existing network for a small business? The building is 40,000sqft and as i said there is already a network in place.Short answer to a short question:
HOOK up WIRELESS routers.
Make sure your equipment has wireless adapters.
Done. (It actually could be that easy)

You won't even need to replace the wired routers, they should all work together, at least somewhat nicely.

If you want a more detailed answer, provide more detailed question please.

-MalFor your needs you would use "Wireless Access Points" instead of a regular "Wireless Router" - The reason for this is that you will already have a router in place for the rest of your network so adding in another can cause ISSUES.  All you would then need to do is configure the access points to have the same name (SSID) and password (WPA/WPA2 key) and connect them into your regular network ports.

The number of access points you will need will vary so I'd recommend getting one at first to work out what the range is LIKE in your building then add more to get better coverage.