Possible? Well depends how you define possible. You can set the AT&T WiFi to except the extremely insecure WEP or WPA1 encryption, and add an original Airport card
(not Extreme) to your iMac, and be limited to 1/3rd to 1/6th the wireless speed normally available on current WiFi routers. Or you can add what is called a WiFi ethernet bridge and get the current WiFi speeds up to the 100 Mbps ethernet speed of your iMac. Ethernet is the slightly wider than a standard telephone jack connector you'll find in the side panel of your iMac. Also you do not need to tear apart your iMac to install the WiFi ethernet bridge. You would with the original airport card. Also depending on the age of the iMac you may also need this adapter to fit in the card:
http://www.welovemacs.com/aicaad.html
So it is really up to you.
USB on the iMac is 1.1, so WiFi USB dongles would be only 15 Mbps per second and slow down to that speed for any other machine on your wireless network.
An Apple Airport Extreme Base Station has a LAN port, and can use that port as an ethernet WiFi bridge for the iMac as well.