Trackballs are relative positioning devices, joysticks are absolute so there's not going to be a simple solder on sollution to this idea. You'd need a relativly sophisticated sollution, such as a micro controller connected to the trackball with a rolling reset or some other method of turning the track balls relative position into an absolute one and a DAC to interface to the controller in place of the POT (joystick) to give the controller the translated position.