Making unit cast custom ability. (GUI)

I'm creating an item that has a % chance to transmute an enemy unit, and to this end I'm planning to have a dummy unit appear and transmute the enemy unit, and disappear. Sadly, the 'order' trigger only accepts preset abilities, and transmute is a hero ability, so I can't give it to the dummy unit without making dummy unit a hero and that will mess some things up. Any other way to do this, or go around the ability block?

An ability based off another ability will have the same order id, so order the dummy unit to cast a modifyed transmute. (which is achived by having an ability with transmutes order ID on the unit)