Why JNB? Other than the mediocre lion park and maybe Nelson Mandela's house and the various museums chronicling apartheid, there's like nothing worth seeing or making a stopover for, IMO.

Anyway I haven't tried this myself (and I can't try cos I don't have the miles in my KF account), but I've read on the boards that for round trips, somewhere during the booking process the SQ website prompts you if you want to add a free stopover.

[edit] Ooh, I was able to try. Just put in your round trip redemption search, then on the screen where you select flights the website prompts you to set up a stopover.