You can change the extension and it will work, but it doesn't alter anything in the file itself, of course. A true .mc6 is quite a different animal to a .pz2 MAT file, because it sets up a proper shader node tree. You can convert .pz2 to .mc6 by applying the .pz2 to the relevant object then saving it back out to the Materials library as a materials collection. Unfortunately, it doesn't convert everything - for example, bump maps are still applied to the Gradient Bump channel, rather than the Bump channel as they should be. Presumably this is for backwards compatibility with Poser 4, as the old-style .bum format maps should, if I've got this right, be applied to Gradient Bump channels.

Steve