Using a servo as a winch requires either a channel with a 3 position switch, and in a modern set this function might be hidden in the software, and not user changeable, or an analogue channel which will control speed and direction of rotation. The best bet in this case is to ditch the servo electronics and use a mini ESC, such as an ACTion Pico, to make sure that there is a deadband. This is after modifying the mechanical bits to allow continuous rotation.