No, it is not possible
The rule you run up against is this:
4(e) Only one intercontinental departure and one intercontinental arrival permitted in each continent except as follows:
1. Two permitted in North America.
2. Two permitted in Asia when one is for travel between the Southwest Pacific and Europe/Middle East.
3. Two permitted in Europe/Middle East for travel to/from/via Africa.
If travel is to/from Europe in both directions, itinerary may not include Mauritius/South Africa.
So you are allowed only one departure from Africa and one arrival
You could end your rtw with ...DOH-ADD
but then you would have no way to get back to southern Africa on the ticket