That would have to be done inside your RC transmitter and it would probably come back to neutral when you switch it off.
The transmitters that I imagine could do this would already have extra knobs.
thoughts:
- for a quad, you don't need a camera roll control, let your gimbal handle that to make it always level
- just use model yaw to pan it left-right so you really only need up-down
- (assuming this is for FPV) use one of your RC switches to control a video switcher and mount a small second camera in the direction you want (probably backwards or down)
- or mount your camera on a servo with the servo controlled by the switch. at least you have two camera positions.
- for a quad, the one gimbal channel you need most is up-down, the quad is not rolled left-right nearly as much as it is pitched forward-backward (up-down) and model yaw works fine for left-right pan (yaw)