From the help section of SolidWorks, only works if you have PhotoWorks
You can modify predefined appearances and save them in the custom folder of the appearance Library.
You can use Pack and Go when you send models with appearances, decals, scenes, or lighting, or you can select Save appearance in the Color/Image tab of the Appearances PropertyManager.
To save modified appearances:
In the Appearances PropertyManager, select the Color/Image tab and apply the properties.
Select any of the other tabs to apply the appropriate properties.
In the Save As dialog box:
For Save in, accept the last folder visited or browse to an existing folder. Typical location is: <install_dir>\data\PhotoWorks\custom.
Type a name for File name. Options for File Locations allow use of different appearances with the same name.
Select Appearance Files (*.p2m) or All Files (*.*) for Save as type.
Additional Custom Appearance Tasks
Delete a custom appearances folder. Right-click a custom folder in the Task Pane and select Delete to delete from the Task Pane and disk.
Delete an appearance. Right-click an appearance in a custom folder and select Delete to delete. This deletes the appearance from disk.
As Chris pointed out, you'll need PhotoWorks to do this--that file extension is used/created by PhotoWorks to display the material's properties.