Hmm, well, try to first book the round trip LOS-JNB with an immediate connection, ask the agent to price it to make sure it comes up at 50k (which, BTW, is an excellent use of miles), then ask him/her to find availability for a stopover itinerary and plead the case that NBO is the most logical stopover point within Africa and it should still price at 50k. I would not use the other reservation as reference- not at this point, anyways. Good luck!