Yes I have been using my mouse wheel to move them but I am a bit confused why you can't map to an axis.
Maybe there is a slight misunderstanding: The second most relevant axis to map in a helicopter (apart from the cyclic controls) is not the throttle, but the collective control which controls the amount of lift the helicopter is supposed to generate. Whenever I fly a simulated helicopter, the axis of my USB throttle controls the collective, not the throttle of the helicopter.
Flying the EC135 starting cold & dark you switch FADEC to flight mode at startup, and to idle/off at the end. While in flight you never touch the FADEC switches (which is why they are guarded by red safety switches).
To lift off you increase the collective power of the helicopter - you do not increase the throttle (well, FADEC does it as it sees fit).
Even if you are in a simulated helicopter without FADEC, it is rather unusual to manipulate the throttle while in flight.