If you find someone that has done this, start with their suggestions.
It is used on quadcopters all the time.
But helis must be set up so that the 'thrust' controls collective, not throttle. I'm afraid that no one has played with that in a long time. It would be much simpler if a single channel controlled both throttle and collective like with a governor? then it would be just like a quad.
For a heli:
System -> Settings -> SystemSettings -> ThrustControl
and press the red up arrow at the top of the screen to save it.
Be very careful. Test thoroughly with motor disconnected / off. Be aware that you might crash a heli.
If you were using a fixed pitch heli the answer would be that it is easy, but that the vertical motion is so slow to react that the vertical PIDs would need to be retuned.