Is it possible to use this to merge a DND Beyond imported sheet (that uses DND Beyond homebrew, and uses the Beyond Import module to import) to a MPMB sheet, that doesn't have the homebrew in the sheet's js? I understand if this is outside the scope of this project, I'm looking for a solution so I can remove my and my players dependance on Beyond, and this would be erfect if so.

Interesting idea. Getting data back in the MPMB sheet will prove technically difficult, if not impossible however.
The only supported way of importing data into the PDF is by importing data from another PDF, as a way of updating to a newer version. The way I extract the data from the PDF is via its XFDF data, which isn't even officialy supported.

I've experimented with feeding the XFDF data back into the sheet, but it always resulted in missing fields and other weirdness. It might be possible with a custom script for the MPMB sheet, but I have no experience with that.

Fair enough, I misunderstood what was meant by "merge an existing FoundryVTT character with a MPMB character!" I thought it could go from FoundryVTT back to the MPMB sheet.

All good though. I'm still weighing up options regarding mine and my players characters, they are currently on Beyond, but I recently cancelled my sub for that, so I don't have access to the content I was previously subscribed to for character data. I have backed up the character data as json before doing so, and I'm coding the js file to enable the data to be imported into the MPMB sheet, with help from the MPMB discord.

I'll speak with my players regarding this, you could have a new purchase in the coming days. And I should ask, this is a module for Foundry, correct? Or is it a separate application that is run on windows / linux