Talk-show host Stephen Colbert is to guest star on the ninth and final season of the U.S. sitcom \"The Office,\" EW.com reported. \"The Colbert Report\" star will play Broccoli Rob, a former member of Here Comes Treble, Andy\'s a cappella singing group, in the Halloween-themed episode to air Oct. 25. Colbert, Ed Helms, who plays Andy, and Steve Carell, who played Michael for seven seasons before leaving \"The Office,\" were all contributors to \"The Daily Show,\" EW.com said.