You need to read the information about the joystick ports, hook up your own potentiometer, and a switch to any one of the digital and analog ports, then program your robot to respond to them on the right joystick port.
For information on wiring, check
http://ifirobotics.com/docs/oi-ref-guide-11-21-05.pdf the pages you want are 5 - 10. Good luck.