Use "Manage Library Materials". Set Type to "Export Materials to Library". Set Target to Single MatML File to have all selected materials exported to one xml file or Directory of MatML Files to have a single xml file written for each selected material. Pick the materials to be exported and OK to create the xml file(s).
To use the material in a different model, use the Assign Materials command. Set the Material List to Library Materials and expand the Libraries subgroup. Toggle on User MatML Library and browse to the .xml file created above. Select your material from the list.