Recently UA/MP begain limiting the redemption of Star Alliance awards for travel trans-Atlantic on LH to 90 days from the date of ticketing. So you may be bumping in to this problem. Note that this 90 day limitation does not apply to FRA-JNB/CPT flights, just across the Atlantic.

When are you planning on traveling?

If you are planning your trip beyond 90 days you will have to use another Star Alliance partner or UA to get between the USA and Germany. UA would be the first choice, then maybe US?, SK? AC? SQ?

I would call the UA/MP International Award resevation number, 1 888 674 4680, and maybe work the FRA-South Africa route first, then have agent try to match up connecting flights from/to the USA. The flights from Germany to South Africa will have to be on LH operated aircraft (57x). Maybe consider a stopover in Europe?

I think SAA operates flights direct, non-stop between the USA and South Africa, so when one can redeem UA/MP miles for travel on SAA this would become an option, provided SAA does not limit award redemption in some fashion?