I'm pretty sure it's possible if you ask, but be very careful if you're checking baggage. It's generally safest to check the baggage all the way through to the final destination, because if it gets lost, they'll send it to FRA and not JNB -- possibly after you've already left for JNB. And I don't imagine they'll let you check in only for the first leg and have your baggage sent to the final destination.

Like the previous poster asked, why would you want to do this? Why not get checked in the whole way and just keep your boarding pass for the second leg until it's time to board?