If u already have the Art for a mech unit, u will need to import those files into ART and XML-> Art .... or at least u will need to import the .nif files into ART (maybe a new folder saying mech), and then reference those .nif files in XML -> Art Defines
Once you have done that, reference the Art Define tag in Unitinfos of XML-> Units ..... Notepad ++ is recommended to assist you in this endeavour