It's only possible to mix one channel to another. The DX7 has three pmixes, but I think you can improve your hopping /falling heli by raising your middle to say 80 and 100 for high, this should keep the head speed fairly flat under load and will isolate stick input to collective changes. I've logged my head speed at 2700 on a fresh bat and have no change pumping between1/2 stick and 7/8, with 430L and 12T pinion.