It'd be truly remarkable if there was a button to convert all of the bricks that you've seen and loaded/rendered OR the bricks inside of a save file, into an interior. This would allow you to create builds that are stuck in a map permanently, unless you go into mission editor and remove these bricks. It would also let you light the build with the maps lights, and if you had placed lights or emitters in the build, these would also appear with the interior, and would light objects as well.
This might seem like a stupidly impossible idea, but it would be, as i said, truly remarkable if anybody could do this.