There are no other available programmable mixers than the one you already know about when you are in heli mode. The software has only so much ability, and the heli software is already using several mixes just to perform the heli tasks. That only left one free programmable mix. In aircraft mode, you do have another programmable mix, but then you don't have the more-complex mixes that are used in the heli functions.

The same thing applies to other transmitters, as well. The 8U has only two programmable mixes in heli mode, but seven in aircraft mode.