Observes the site, "Serkis will join returning cast members Cate Blanchett and Elijah Wood. Wood's participation was made official last week after Deadline first revealed his negotiations. According to sources, others who appeared in The Lord of the Rings poised to return at this point include Ian McKellen (who's still working out a deal to return as Gandalf in a major role), Christopher Lee is in talks to reprise the wizard Saruman, and Ian Holm to return as an older version of Bilbo Baggins (Martin Freeman is playing that title role in the two new movies)."
It is, of course, Gollum who introduces the ring to Bilbo, thus setting up events that take place in the Lord of the Rings trilogy.
Of Serkis, who will be portraying Caesar in this summer's Caesar: Rise of the Apes, Deadline notes, "Serkis will likely be there early in the shoot. The chameleon actor, who has become a fixture in Jackson's movies, will do reshoots for The Adventures of Tintin: The Secret of the Unicorn. Serkis plays Captain Haddock in the film, a collaboration between Jackson and Steven Spielberg."