Wait now...
If there's a will, there's a way. Along with a lot of time, research and soldering.
I'm envisioning a desktop PCI/USB 2.0 card being soldered to the PCI pads that support the airport card. The Airport card is PCMCIA, which is the mini form of PCI (and has adapters available to switch between them). The Airport card uses a breakout board to plug into the logic board of the iMac, so it's just a matter of figuring out the pins that need to be soldered and soldering them to jumpers and to the OS X supported USB 2.0 PCI card.