Is there a high level view of how to implement custom motor control?

I have a rover that has its own motor driver board, 4 motor skid steering. It has an RS-232 protocol for controlling the motors and getting motor position details etc. How do I go about talking to it?

Do I modify the ArduRover firmware to interface with it, or do I attach a companion computer so that all the logic is separate? Is there a high level overview of this process or some examples I could read about?