In order to load multiple master files in the G.E.C.K. (to allow your mod to make use of multiple DLCs, for example), a simple change must be made to the GECKCustom.ini. That file is found in /(My) Documents/My Games/Fallout3 (Or FalloutNV). Simply open it up and change bAllowMultipleMasterLoads=0 to bAllowMultipleMasterLoads=1, then save. http://galahaut.blogspot.com/2009/11/how-to-load-multiple-master-files-in.html