Creating MAT files in Poser is easier than you think.  Load your figure/prop into Poser, enter the Material Room and manually dress it with your texture.  Unfortunately, you will need to edit each material zone to apply the textures.  Include any nodes necessary.

Once done, click on the add icon at the bottom of the Poser Library (+ icon).  A new panel will appear asking whether you want to create an MC6 (material collection) or an MT5 (a single zone material).  Select MC6, then select the material zones the new file will affect.  Give it a name, click ok and you are done.  Load a new figure to test.