There are some options.
Option 1: Hollow out
this pinion (or a gear with the same specs) that mates to this
AndyMark Planetary Gearbox, then modify the gearbox to add the mount holes for the RS775, then put that gearbox anywhere you can put a CIM. Read
this, all of it. You will need some pretty decent machining capabilities.
Option 2: Use a CIM on the arm to test. Then wait for Banebots to get it together and replace it on the Thursday of competition.
At either of those points, however, it may be easier to use the FP motor if you're only using 1 motor for the arm. Given that you need to gear anything down by quite a bit (1000:1 or more) in order to easily control the arm (2 seconds per 90 degrees), you could probably get away with that. With such a long arm, the manipulator will have to handle the forces of accelerating to and from over 10 feet/second at the tip.