Things have changed and if you want to get the ATP, You need to go to the SACAA and get them to send your info to the Feds/ FAA. Then you contact the Feds in Oklahoma to verify they have the information they require. Once this is done, 30-90 days, You choose a school, complete the TSA requirements, obtain a visa and off you go to the USA to do the test. However, if you do not have an ATP already they will not necessarily let you do the ATP as you FAA certificate will be "based" on your South African one, even though you have passed the US test, and will only be valid if your SA license is valid.
The best way to do it if you meet all the requirements for an ATP is to study and write both the FAA Comm and ATP, and do both the flight tests, this way your FAA Comm is based on your SA Comm, but the FAA ATP will be stand alone as it is based on your FAA Comm. Please also note that the FAA ATP requirements differ from the SA ones.
The writtens and flight tests can be done in the UK, Frankfurt or the USA, but you need to complete the TSA requirements and license verification, even if you choose not to go to the US. (Europe will be more expensive, probably by double)
Please feel free to PM me if you need any further info, I am a current FAA CFMEII in SA.