You can get a Comm validation on the back of your "frozen" ATPL, but that only lasts as long as your JAR is valid for (medical/LPC etc).
You cannot validate a foreign instructor rating. An instructors rating requires a SA CPL, 20 odd hours of instruction, and a couple more exams.
If you have the JAR, you will find the SA exams pretty straight forward. My advise is get the JAR, come to SA, sit the exams straight away, do the flight test, and get the SA CPL. If you get the SA CPL first you will struggle with the JARs.