The short answer is yes. the long answer depends somewhat on whcih iMac you have.
All iMAc 400 mhz and above (and some 350 mhz iMacs) have an airport card slot. So if you can get ahold of an original apple airport card and an airport card adaptor, you can just use that. Airport cards aren't made anymore (airport extreme won't work), so they can be a bit pricey and hard to find. lacomputercompany.com use to be a source of them.
The second option will work on any iMac, actually, on any computer period. You can just get an external wireless bridge. Its best to go with an ethernet based one. USB bridges need drivers, whose support for macs range from flaky to non-existent. Ethernet doesn't require drivers. I use a linksys WET11 myself for this purpose, works great. Linksys also makes the WET54. Belkin, dlink and macsense also make such adaptors. The downside to these adaptors si that they are external, so they take up a bit of space, and they need to plug into a power outlet (except the macsense one, whcih uses USB to get its power but connects via ethernet for the networking side of things). You also will see many of these adaptors listed as game console adaptors, since they are commonly used to get game consoles online wirelessly.