If you want to get there in style, it's hard to beat CX F USA-HKG (optional stopover there), then CX J HKG-JNB (they no longer serve F to SA), all for the low price of 70K AS (transferrable from SPG).
AS also partners with Emirates (EK), though the rate to Africa is higher @ 100k for F, and there have been problems booking those (F). You could fly J for 77.5k.
EK's own award chart tends to be overpriced, though I haven't checked their rate to Africa. They also levy fuel surcharges, so I would look elsewhere for redemption.
Star Alliance has multiple African carriers, so that's another place to look.
What if I convert them and the flights end up being taken?
Then you're stuck with a bunch of miles in that program. It's a risk, and it would be good to have a backup plan (e.g. what else can you use the miles for)