Changing the outfits that zombies wear is fairly straightforward. In the editor, open up the Ambiant [sic] Zombies module. You'll notice at the bottom, there's a box marked Zombie Uniforms Override. In this box, you paste the uniform names and they'll be added to the zombies. Obviously, you will also need to load the mod which you're taking the uniforms from.
Evil Organ has made an excellent video showing how it's done. He has also compiled a list of uniforms and their names for use in the editor. You can find it at the following Dropbox link.