balance systems are about adding thrust to body parts to counter-act gravity. Skateboard is put together in a similar way to a real one, you need a board, truks and wheels. The wheels attatch to the truks, and the truks attach to the board. You set the ranges of the joints so that when the board tilts left or right, the wheels will angle to turn in that direction.

As far as modding goes, it'll be very difficult for a begginner. Try something simpler to begin with perhaps.