Patch, but why the serious enemy packs downloaded from internet cannot be added to more than the original ones?

Downloaded to put enemy packs for more enemies.
 But in the editor, so for put more of those enemies around, cannot insert any new enemy, that if there are some new, it just overwrite the original ones. It always say missing in entities.dll whatever, even if there are other customentities.dll made. 
what to do to add more enemies.