They dont show up in the drop down? If so you may have to select them in the browser and press the Left arrow next to the field. Sometimes saving and reloading the engine will make them show up in the list. Not sure why this happens, I bug reported it a while back and never heard anything. Usually after you manually add it through the content browser they show up forever.
Ahh gotcha, sorry I guess I wasnt clear. Make children of the base classes, duplicate already made children. Sorry I made it confusing :(.