[ChrisH]

Most R/C radios these days have a provision for what is known as a "buddy" box. Usually it is a round connector something like what they used to use for a mouse or keyboard. The idea is that an inexperienced pilot can hook a friend's transmitter up to his. If he gets in trouble, just flick a switch and the buddy can take over from his own transmitter.

It should be possible to build an analog of the buddy transmitter and plug it in to the existing port. Also putting the transmitter in "buddy" mode could be a good way to figure out just what signals are required.