Using a custom control box in addition to the drive joysticks is a great idea, we are also building one at the moment.
The pinout diagrams for the OI ports are available from IFI in the OI reference guide (they have not changed since 2005/05 I beleive) -
http://www.ifirobotics.com/oi.shtml
The pinout chart gives the pin number for the OI digital and analog inputs, as well as the LED feedback drivers, and the IFI aliases for those inputs/outputs.
You will need 15 pin solder cups [male and female] and some basic wiring skills, but its quite simple to do. For switches - wire them between the input pin and any ground pin of that port.
good luck, have fun.