Actually, the stuff that Jackson is adding is mostly there in the background already. In the original story of The Hobbit, Gandalf leaves to help the wise drive out the Necromancer, which is really just an excuse to get him out of the way so Bilbo can grow. The later appendices and Silmarillion show how Gandalf and the White Council gather together to decide what to do about the Necromancer, whom they suspect is Sauron, and they decide to take action and drive him out.
As for LotR characters, there is no reason to think that Legolas isn't present when the Dwarves are captures. It's his dad who takes them, after all. And Saruman and Galadriel are, as mentioned above, part of the White Council.