The easy way to make a mat file in Poser 7 is to load the figure and apply the material you want to save .  This doesnt work for earlier poser versions , because they cant save a mat group.
then in the material room , save a material group to the mat library, name it something meaningful.
then copy the xxx.mc6 file from the material library folder to the pose folder and rename it xxx.pz2
then open it in notepad and change the line that reads mtlCollection to figure  - thats it - you are done , I like to make a nicer thumbnail for it and add text so its obvious its a mat pose.  poser uses 91 x 91 png for the thumbs in the libraries.

xxx.mc6 is whatever you named the file. you can do the same with an mt5 file. mt5 is ONE material group, mc6 is multiple materials.

hope this helps.