They are adding things. In the book Gandalf would fuck off for no reason, and we only get snippets of what he was doing in various appendices. They'll expand on it in the film, with him, Radagast and Saruman meeting, presumably to discuss the Necromancer of Mirkwood. The banishing of the Necromancer with be in the films, though I'm not sure where. In the book it happened several years afterward, before Gandalf and Balin came back to visit Bilbo.
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