First of all, I think this post isn't in the right thread. I assume it should be PowerExchange
My own experience with PowerExchange for SalesForce is limited and from a few years back.
What I do remember is when importing the structure from the SalesForce object into PowerCenter all the metadata of the fields was imported. So in this case in PowerCenter it was visible which field was defined as the external id etc and PowerCenter is using this information.