Right but there is some way you could spawn a mechanical critter with a little bit of triggers. Catch when a unit of the right type enters the map and have it or some other unit cast the mechanical critter spell at its location, then remove the original unit.
I am unfamiliar with the specifics of the Mechanical Critter spell/item, but pretty much any base WC3 ability can be used on a unit, item, or hero as you desire. It originally being an item ability is not a limitation.