Ha! There is actually a clever hack for this!
If you copy some notes from one project to the other and it realizes that the instruments are missing (it will matches them by name, so careful!), FamiStudio will ask you if you want to create the missing ones. Just say yes, and boom, you have your instruments!
This was covered in a previous tutorial (that I dont recommend watching, its pretty bad!). Here is the clip:
-Mat