They use them as costume , beacuse to get "transup" working u need to edit exe file, because clients reads from skill file ActionType,PREPARE , and /scrpit/shapeshift but there is more stuff like property/effect...etc.
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I forgot to add..idea come to my mind ...maybe it can be done by editing "emotions" like sit..etc and then change their commands , i think u can change char look also ...well just make it "transup" but thats not rlly ...original
just an idea u can try it T_T