You could hack it electronically, both PS/2 and USB use the same 4 inputs/outputs: Voltage, Ground, Clock, Data. The problem is that there's no universally accepted pin layout, which is why PS/2 to USB connectors don't work unless the keyboard supports USB as well anyhow, hence the need for fancy PS/2 to USB adapters.
If you have an old keyboard like say an IBM Model M, the electronics and pins inside it are well documented so you should be able to accomplish it with few problems. However you'd have to get some fancy footwork on the go with a soldering iron depending on your motherboard if you want to make the green PS/2 port working with a keyboard.
One way to do it, if you wanted two keyboards at once as I do, is to unsolder that green port and connect it pin for pin to the purple one - that's what I'm gonna do, although I'm doing it slightly differently - sticking the port in the back of my UNICOMP Terminal Emulator and wiring the 4 pins up on the circuit board so I can plug my IBM Model M into it :D
~ All must bow before "the one true keyboard" and its perfect, super-sonic buckling spring design! ~ Rob ~