Via Huffington Post: It was announced in March that Will Arnett and series creator Ricky Gervais would guest star in the finale, while earlier in April Ray Romano, James Spader and Catherine Tate were added to the episode. All five have been teased as potential longterm hires, though Arnett, Gervais and Romano, because of previous commitments, are unlikely to be the choice. Ferrell, who was supposed to appear in the season finale, will now no longer do so.
With Carrey being added as a late entrant, the future of Dunder Mifflin gets even murkier. Executive producer Paul Lieberstein, who also plays Toby on the show, recently revealed that current employees will make a play for the job, though it doesn’t seem there’s much of a chance that they’ll be hiring from within.