The Airport card is not a standard PCMCIA slot. It will only take an Airport card and a very few other Airport like cards. It cannot and does not do anything else.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.
The later G4 iMacs have USB 2.0. The 1Ghz 15", 1.25 Ghz 17", and the 20" all have it. The other models all have USB 1.1.It might just be cheaper and less time consuming to buy an used iMac G4 or a similar computer
EDIT: probably not cheaper, but obviously faster. Still, G4 iMacs are not expensive at all.
EDIT2: I should just erase this post. xD They don't have USB 2,0. iMac G5 have it, though.
Don't mean to sound like a jerk, but a PCMCIA slot is a PCMCIA slot. Just because an airport card fits it most easily doesn't mean it wouldn't work with any other card. An Airport card is a Lucent Wave Gold card without the built in antenna. Don't limit yourself, IntellThe Airport card is not a standard PCMCIA slot. It will only take an Airport card and a very few other Airport like cards. It cannot and does not do anything else.
I've tried this and a SD card reader. None of them work. That's why I'm saying: it'll only work with Airport cards or Lucent based one.Take a PCMCIA USB 2.0 card and try it out! You can remove the bottom of an iMac G3 pretty quickly with a few phillips screw drivers and then mod the iMac Airport Card Adapter to accept the card! If it all works out, the case should go back on over top of it and you can run a couple USB extension cables out of it!