As far as I know, characters such as Leia, Luke, Vader, etc. can't be added, nor can copyrighted stuff such as the Star Wars theme ripped straight from the movies, but anything else can. So you can't rip the Battle of Endor from the movies and include it in a cutscene, but you can perfectly well recreate it from scratch using 3D models, as long as you don't include hero characters.
/layman understanding
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.