I'm not at home so I can't really check but I don't think it's possible for a hero to build. If it is possible you would have to give him the build abilit(y/ies) of a peasent/acolyte etc.
I dont really know but...ive tried changing the item of making a building and thinking of making a trigger that gold is taken away from whoever builds...also i dont really know if build does work...i want to make the same ability of build like the one in Nature's Call - Circle Defense.
Heros cant build Buildings but you can use Spells! U know this Item in the Humanshop to build a Tower. This is a Spell! Make a new Spell base of this an set the Building to you Building! Then give the Hero this Spell to build this Building!
But I do not know whether it functioned there I invented the degree of all out of the head! But it would have to function!