It all depends on what you want to accomplish.. If you want to use computer control over a network, then you can do this many ways.. You can make your own link and taylor it to the circuitry integrated in the car, or you can design your own interface to run the motors.
Alternatively, you can use some digital potentiometers in place of your controller's mechanical potentiometers and use their interface. If you want a quality transmitter that you can add other controls to, using your own link is recommended. Usually when people use ready-made platforms they are just wanting to save time making the structure and etc., rather then replacing the electronics completely.
As I said, you could study the PCB and the signals going to the board and make some cuts in the board and replace the signals with your own. I would probably just make my own interface since it isn't too complex.
Steve