OK, so we lied. There is source code for a Forte4Java implementation, but someone else wrote it and it's kind of old. It *should* still work, but there are no guarantees.
First, you'll have to build Transmogrify with Forte4Java support enabled. Read the Building from source section of the User's guide for details.
Once this is done, place lib/transmogrify.jar wherever Forte4Java expects plug-in jars. I really don't know where this is, so good luck.
You might also try looking in src/net/sourceforge/hook/ffj as there appears to be some Forte4Java specific documentation there.