For JFK-JNB and vv on SkyTeam metal, the best options schedule-wise are probably DL via ATL, or KL via AMS. AF via CDG is also possible (but with a long transit outbound). You can even go on SV via JED, but that has an overnight transit outbound, and outbound is only possible once a week (as SV only fly JED-JNB twice a week, and one of those flights would require a transit in JED above the 18hrs allowed for TWOV...)

However, I don't have any insights into SkyMiles Award availability on any of those routes...