You could do it a few different ways and im sure the devs could think of better ways then me.
Could either add it into the VIP thing, you could have a building (similar to the barracks) where you can upgrade each builder seperately and assign those upgraded builders to each thing your doing at that time, or maybe just a building where is a way to get a builder of either , Basic, experienced or elite builders and you make them like heroes and the 5* builder would be the best of course.
I havent thought about it too much just come to me as i was typing this haha.