But you don't need anything nearly as complicated as Rocksmith files or CustomsForge to import songs into FATpick. FATpick already accepts Guitar Pro files, so there are thousands, maybe 10s of thousands of scores you can download from ultimate-guitar and other sites to import into FATpick.
Also, while obviously not as convenient, free software like TuxGuitar can convert other formats (MuseScore, MusicXML, etc.) into GuitarPro, so you can import pretty much any machine-readable score into FATpick (modulo proprietary formats like Rocksmith/CustomsForge, but you need to jump thru hoops to "extract" data in that format in the first place).