Yes you can.
It's actually not that hard at all. Lay the iMac down with the bottom facingyou and the screen face down on a towel on the table. Obviously, ground yourself and make sure everything is unplugged before you unscrew the bottom circular panel. The screws are spring loaded and stay with the bottom panel. So if they don't come out, their not supposed to. Once the bottom panel is removed, the Airport card slot is right there dead center with an extra RAM slot. Insert the Airport card (make sure it goes in ALL the way) and connect the antenna cord to the top of the card. Close er' up and you should be done.