They have the rights to anything in LOTR and the Hobbit. Much of the proposed "extra material" is in the Appendices to LOTR--most of the Arwen subplot in the movies actually came from here. But they can't use the Unfinished Tales or the other books.
Right now I assume they'll just incorporate it back into the Hobbit--e.g., showing what other characters, such as Gandalf, were doing "offscreen"--and fill two movies that way, but I'm really not clear on it yet.