Independent horizontal and vertical gain

During different stages of a dive, I need to control the vertical thruster gain differently than the forward thrusters. I would like to have vertical-gain increment and decrement operators assignable to buttons on my controller.
Is this an existing capability that I have overlooked? If not could anyone point me to where I should be looking in the source code to adapt it? (I’m a programmer, but new to this code base).

Thank you

This is not possible.

Re the source code, this is where to start:

This is the mavlink message used to communicate the joystick state to the autopilot
https://mavlink.io/en/messages/common.html#MANUAL_CONTROL

This is the handler on the autopilot