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