Create then generate terrain?

So I hired someone to make a map for me and already he has hit the maximum part limit. The map takes place on a mountain, so you can get the idea of where those parts are coming from. He said it would be easier if the mountain was built out of terrain, and I'd be good with this, but there are two problems.

1. After looking throughout the internet I cant seem to find a way to get any data on terrain that you make using the editor(ex position,size,etc), like if I wanted to generate this terrain set through a script.

2. I don't if you can easily generate terrain you first made with the editor.

The only other way would be if he were to go in and painfully make a script to generate each part of the terrain one by one.

So all I'm asking is if something like that is even possible.