We've never had Design Exploration Extension, but a quick look seems to indicate that it's designed to let you accept the current state at a "checkpoint" which will be applied to models in the Creo session. The TMU files, whatever they are, are not meant to be "converted" to a .prt file. The .prt file is modified by applying the changes you want to "accept".
Again, I haven't ever used the thing, I'm just relaying what I read about it.
