Yes, you could do either:

NA-SA-EUR-AFR-AUS-ASIA-NA
or
NA-SA-EUR-AFR-ASIA-AUS-NA

With the former requiring QF64 JNB-SYD (only MWFSu) and the latter requiring CX748 JNB-HKG. Note that CX748 does not offer F outside of the SA Summer season, i.e., past March.

BTW, I don't think the 6 continent trips are that worthwhile. IIRC the price jump from *ONE5 is meaningful, and since you don't get any extra segments (20 max), and 6 are used up for the intercontinental segments, that's only 14 segments left for 6 continents, barely more than 2 per continent on average. On the ONE5s you get 15 segments for 5 continents, or 3 apiece on average.

Unless you don't mind purchasing a lot of extra segments on your own, or you only want to visit 1 or 2 cities per continent, the *ONE6 isn't a great choice. If this is a mileage run, I'm sure there are ways to get just as many on a *ONE5.