if they can already check you in for the return flight upon leaving the States, it might very well be possible. Upon arrival, you'll come to a hallway where you either turn towards customs/immigartion or go up a set of stairs to the transit area. Of course you will still have to pass a (number of) security check-points on the way to the gate.
However, there are two reasons why I recommend that you contact your airline first, telling them your intentions and the reason, and asking whether it is possible to check in for the return flight as well. Firstly, I do not know if your unusual itinerary will cause questions anywhere, and secondly, if you cannot already check-in for the return leg, and should then - for whatever reason, e.g. a delay on the inbound leg - not show up at the FRA
check-in desk in time for your return flight, the system might consider you as a no-show...
So, better save than sorry, give'em a ring! Cheers, Lukas