It is not possible to assign to specific Program numbers to aux switches. This is a job best suited for a MIDI controller. However, this is exactly why the H90 has Bank mode, where each bank on the H90 allows you to access three programs at a time, one per footswtich. Have you tried Bank mode?
Bank mode video: https://youtu.be/5y14LwN9oAk?t=571
For future reference, you can find all assignable functions for external aux switches here: https://cdn.eventideaudio.com/manuals/h90/1.7.5/content/appendix/global.html
