Adding new buildings and scenery: for now you can't do this. You can only add new placeables, npcs, doors, etc and reskin an existing area.
Can I half agree with you here Darth333? There was this one thread awhile back where one of our newer members blawk was showing off. Now there were still limitations and he hadn't gotten it 100% down but it was a start.
Let me see if I can find it. Ahh here it is ->
http://www.lucasforums.com/showthread.php?t=164603)
Hey you were even following this one. :D
But seriously this is why I say 1/2 agree nothing more. ;)