/vr/ - Retro Games

Im making my first doom wad but i have a problem. I can download custom monsters and edit their appearance etc and I can of course take and edit the sprites of the default monsters. But I dont know how to input the custom normal monsters. Slade just says I cant save iwads. I can replace the customs monsters appearence and use them but how do i replace a normal imp for example?
I guess I need to make a copy of the imp but with a different appearance, but how do I get this monster to have imp behaviour? The custom monsters have this "decorate" behaviour file. I can make custom-custom monsters, but not custom-normal monsters.