Hi! I haven't used Book Fusion, and from its home page, it's not completely clear to me exactly what it does. So I'll answer in a general sense, then you can provide more detail if my answer wasn't good enough.
JetReader (right now) does three things:
1) It's a reader app that can be used to read the most common eBook formats.
2) It's a library app that can show your books, and let you pick a book to read (and open it in JetReader, OR in another reader).
3) It's a Calibre synchronization app, like Calibre Companion or Calibre Sync) that let's you download your books from Calibre to your device.
So if you want to use JetReader to handle Calibre sync, then open the books and read them in Book Fusion, that's already possible.
If it's the other way around, that you use Book Fusion as a book server, and would like to use JetReader to sync these books to your device, then read them in JetReader (or another reader), that's not currently possible. If it's a wish, however, I'm very open to adding more integrations, so I could implement a sync-feature towards Book Fusion as well.
Let me know if that helped!
(Also, I've been silent in this thread for a few months now, because I'm working on a huge overhaul of JetReader, moving it over to another more modern framework in order to simplify future development. Will launch this update quite soon now!)