Well, I tried to post this last night, but Netscape froze up. Anyway...
If your computer has PCI slots, you can get a PCI card that will give you USB ports. I believe that you have to be running OS 8.5 (so, 8.6) for this to work. I have never heard of a similar product for PS/2 peripherals - what would be the market for that?
There are adapters for using ADB on a USB Mac, but I don't believe there are adapters to go the other way, either for USB or PS/2.
To find actual products, search this and the G4 and iMac forums for "USB" and "PCI" - I'm sure you'll find some good links.