HoMM VII Haven Units – Haven faction units from the seventh installment of Heroes of Might and Magic.

You can elaborate on what you just wrote. Although, to be honest, I still don't understand if you managed to create a file that allows the map editor to work with Max, or not. Phrases like "it crashes with an error until I manually add it" make it a bit difficult to understand the text. Where and what did you add? The material in Max? Or somewhere during import? What do you mean?
By the way, if it works, it would be great if you could share a link to the plugin that exports GR2 files from Max.

By the way, do you feel like messing around with models from Heroes 6? Re-rigging them to the skeleton from Heroes 7. I was messing around with it a week ago. In principle, it's not that difficult if you get the hang of it. You can either re-rig them manually (it's worth watching two or three tutorials on rigging in Max, and you'll already be able to do it). Or, as an option, you can use the export from Noesis; the model will then be exported with a mostly working rig, but the bone names and sometimes the skeleton structure will all be incorrect. Theoretically, you just need to transfer the ready-made rig (Max has this function) from the broken skeleton from Heroes 6 to the working one from Heroes 7.