Yes, it's theoretically possible if you use fully flexible fare buckets that support this. It will not be cheaper, however.
The deal you've found is for LHR-JNB and it's not available on ABZ-XXX-JNB (where XXX happens to be LHR) any more than it would be on OSL-JNB, WAW-JNB, etc.
This JNB holiday deal is so good that I would personally book it from LHR and extend my journey to allow plenty of time in London.