He's already said he is drawing from the apendices at the end of ROTK. Its like 125 pages that tolkien wrote to fill in some holes. It covers various times, events, people, amd places. Jackson used material from the apendices in the LOTR trilogy. At this point these new films are more than just the Hobbit book which is fine. If the apendices arent adapted at a time in the future the rights revert back to the estate.
This has been discussed on the previous pages and jackson has made a few statements about this. At this point he would like to shoot more material to help the hobbit flow directly into LOTR. His statement was that "if somebody wants to sit down and watch all five or six films in one sitting they will flow seamless into one another like one very long film." This possible third film will bring these two film into seamless sync with FOTR. So essentially the third film will be a bridge film. Or they will expand the second film into two films and add the extra material into both.
I wonder what they will call the bridge film? If its purely a bridge film and the Hobbit story ends in film 2 the third film cant be called the Hobbit since it wouldnt make any sense. Someone in here suggested The Chronicles of Middle Earth. Anyone got any other ideas?