Indie game storeFree gamesFun gamesHorror games
Game developmentAssetsComics
SalesBundles
(+1)

This would be awesome to have as a tool for the Unity 3D game engine. How difficult would it be to do that?

Ah, I see! I've just answered "kind of no" to your question about using non-default buildings, but in the context of Unity 3D it is possible I guess.  Unfortunately my knowledge of the engine is virtually zero...

(+1)

There are a bunch of courses on Udemy, any one of which could teach you what you need to know.

https://www.udemy.com/courses/search/?q=unity&src=ukw