What you are after is a USB Hub.
If you are looking to plug in all your mice, encoders, etc... into something via USB, and then only have two cords coming out of the CP (USB cable, and power cord), then a USB hub is what you want.
I'm not even sure that it has to be a POWERED hub.
I'm using a cheap non-powered one in my cab, and haven't run into the need for a powered one.
Just use the foam tape to hold it down inside your CP, and plug everything into it, leaving the USB cable (and power cable, if you go that route) coming out of it somewhere.
The advantage to using a USB hub over the PS/2 cables is that you can hotswap it.
This would let him have a keyboard plugged in, and then plug the CP to the front USB port when he needs it, and unplug it when he doesn't.
They do not recommend hotswapping PS/2 cables.