To give an actual example of what markschum said. If you open a MC6 made in P6, the beginning of the file will look like this: {

version
        {
        number <span style="color:rgb(255,0,0);">6</span>
        }
<span style="color:rgb(255,0,0);">mtlCollection</span>

Edit it to look like this: {

version
        {
        number <span style="color:rgb(0,255,0);">5</span>
        }
<span style="color:rgb(0,255,0);">figure</span>

Now save it to a pose folder with a PZ2 file extension. The simple procedure of changing "mtlCollection" to "figure" converts a MC6 into a classic MAT pose file. But note that there may be new types of nodes in P6 and later versions of Poser that are not supported by P5.