Hi Tafigand
I don't know if the reply is still as valuable as it was supposed to be a year ago?
You don't exchange your American for a Zimbabwe driver's licence you keep yours a you got for a test to familiarize yourself with the Zimbabwe Road Rules which are pretty much the same internationally except that in USA you drive on the right side of the road.
So the test you go for will basically enables you to get an international driver's licence.