This belongs in the WAE General Discussion section moreso than anywhere else, but I can try and explain briefly.
Adding a custom hat texture to the game is
relatively simple, if you know what you're doing. If you go to the Editor root directory, then from there to Data/Models/Stinker, you'll see a lot of different files in different formats, mostly .wd3 and .wdf. The .wd3 (and .wd1) files are models - while it's not
impossible to add a custom hat model to the game, it's much trickier.
The .wdf files are the hat
textures themselves. Adding a new hat texture for the
player to use is simple - just save the texture in that folder (I would save it as a .bmp to be safe), change the extension to .wdf, and rename it accordingly. While there's
likely an upper limit on how many textures a single hat model can have, we don't know what it is.
Making an NPC use a custom hat is a bit trickier - You'll have to modify the NPC object in either a .wop file or in the .wlv file itself, to make the hat colour value point to your new texture.
To explain it very briefly: Using a hex editor, you'll need to change the value of Data3 in the NPC's data. At some point, I'll try and write a proper tutorial about how to do this, but for now, you can find notes I made on the layout of .wop files
here.
I hope that helps somehow.