Dan from 4ms posted some information about the button expander a few weeks ago. It’s the next module in line for release. One item he mentions is that the buttons will not have any lights.
Button Mapping through Expander - #4 by danngreen?
He mentions in his post that “When you make a button mapping, it’s the same as when you make a MIDI button mapping: you can choose between toggle mode or normal”. I use a Novation Launch Control with my MM and it has buttons that can be mapped to MIDI notes. I use the buttons on the Launch Control to toggle plug-in switches.