Adding material presets is easy.
Create a new virtual directory or select existing one. Right-click the context menu item from where you want to add the material:
a) Get Material From Selected Object
b) Get Material From Slate Material Editor
c) Get Material From Compact Material Editor
Done. Materials is added to the selected directory (Category or Folder).
XSMP will automatically create a new directory for material that will contain the material file (.mat) and preview images.