Cathaybrat,
A few things to look at, on the validation, you may fly anything that is "in your log book" ie anything that you are legal to fly on your FAA ATP.
In order to add a type rating, ie anything over 12500 lbs or and turbo-jet, you will have to do a FULL type rating at a FAA recognized facility. Furthermore, no type rating issued in South Africa will be transferable to your FAA ATP. as there are no FAA DE's in South Africa.
Basically if the aeroplane you are hoping to fly requires a type rating under FAA regs, you have to do a full FAA type and have it on your FAA certificate.