I think what you search for is in Part 61.
http://www.caa.co.za/What's%20new/CI...TIONS-2011.pdf
See bottom of page 242 of this long download. All the information is contained therein but the gist seems to be:
If you've kept an overseas commercial licence current you might apply for a dispensation from the Commissioner for Civil Aviation, submitting a motivational letter, certified logbook entries and foreign licences with check rides. If you get the dispensation you will almost certainly have to write Air Law and Flight Procedures, and they're likely to only give you one shot at that.
You'll also have to pass the relevant flight checks of course.
If you haven't kept a foreign licence current then I suspect you will have to rewrite all the commercial subjects again and pass the flight test as appropriate.
The licensing road ahead is unlikely to be an easy one, good luck.