Electrical objects won't automatically attach to roof soffits.  If you would like them to, then you should probably submit a feature request.   For now, I would just place a ceiling mounted light from the library and adjust the height so it is positioned properly.  You may also need to use the Open Symbol tool to turn off the "Flush Mounted" option so that it is flat and doesn't try to follow the slope of the underside of the roof.  You should not need to create an extra ceiling plane.