you have to open the modellin wc3 veiwer export it's skin, convert the skin to *.tga open it in photoshop run the alpha channel and paint everything black you want to be invisible in game. than save the skin as *.tga convert it back to *.blp and import it exactly with the path of the original skin. save map reatart WE and that itshould work.
Make sure the *.tga is 32 bit and uncompressed before convert to *.blp...