It isn't quite as simple as just changing the file extension though, just a couple of seconds extra work and then you are set.

A .pz2 extension file needs to have {figure} after the version number "slot", so if changing a material collection (.mc6) to a MAT pose file (.pz2) then this needs manually editing - trust me forgetting to do this means you can double click 'til your mouse falls in bits but that texture will not apply to your cloth, figure or fancy shoes.

Similarly the .mc6 file extension needs to have {materialcollection} after the version number "slot" or it brain farts on you.

Spot who bought a MAT pose generator only to discover it is so much quicker and easier to save out as an .mc6 and then convert to .pz2 for P5 users.  That is why I include both .pz2 and .mc6 in a pack, my shaders and other bits and bobs will work just fine in P5 but as it can't understand a material collection file this would mean restricting my potential audience to P6+.  Plus some users prefer the good old MAT pose file no matter what version they are working in.  In my case the information contained within the .pz2 and the .mc6 are identical.

INJ/REM poses will not work from the material room as they do not change the appearence of the texture but change the appearence of the underlying mesh.  This is why you have noticed that although the skin, eye and makeup options for a figure can be present in two folders - one set for .pz2 and one set for .pz3 - the actual character morphs are only in the pose folder.