I would do either of 3 things
if you have multiple throttle levers just unassing one and then assign that one to collective
or I would make another joystick profile (at bottom of the screen and normally says user profile when in joystick settings in xplane11) and only include helicopters in it and unassign the throttle and assign that axis to collective then when you fly airplanes you will have your throttle and when you fly helicopters you'll have your collective.
or just unassign the throttle axis and assign it to collective
Let me know if you need clarification
hope this helps.