jass has nothing to do with it.
I remember reading somewhere that it *is* possible to make 1 model use different skins, but don't ask me how...
The easiest way is probably simply importing 2 times the same model with a small mdl edit so they use 2 different skins. That will, however, require an additional modelfile to be imported in your map per skin.
And like people have already told you, serpent, it's all in the model. The SAME modelfile can have multiple variations, but it's only "faking". Blizzard too has 3 different models for the city building, and a 4th "model" which contains the 3 original models as variations. I don't know how to do it but it's pretty useless to do it yourself IMO... It'd be easier to simply copy/paste your doodad and use 2 different models.