If motor guide is using PWM (pulsed DC) to control motor speed then they are just varying the duty cycle to the motor. More on than off to increase speed. You could probably replace 5 speed switch with variable resister or rheostat.

This link gives a pretty good overview of PWM. Pulse Width Modulation Used for Motor Control

I googled "motor guide PWM speed control" and quite few solutions to what you want to do came up. Looks like a few on CDC have solved same problem before.