If you want to mash down on piano keys and have PARTs 9-16 respond to different regions of keys you press, it's not going to happen.

The primary function for PARTs 9-16 is for external keyboard control. So if you want those PARTs to respond to mashing keys - they have to be keys located on an external keyboard. Maybe another Montage - maybe some other 8-zone controller (RD-2000, Kronos=16 zone, Kurzweil most are 16 zone, etc). Or you can use a computer to play those PARTs through MIDI. Or you can get a programmable MIDI router to translate key ranges and target (route back to Montage) those PARTs 9-16.

Some more info:

https://www.yamahasynth.com/forum/montage-as-master-keyboard-explain-zone-relationship-to-montage-parts
or
https://www.yamahasynth.com/forum/to-bad-mister,-a-little-more-on-the-performamce-8-parts-1-of-8
or
https://www.yamahasynth.com/forum/can-t-access-parts-9-16-except-as-solo

If you run out of PARTs - you may be able to re-arrange your existing elements inside PARTs and better "pack" them so you free up PARTs. Most PARTs do not use the 8 elements available so you can "merge" two PARTs under certain conditions.

Thank you.