Spader is a former 1980s film star whose career was revived by a starring role as brilliant, manipulative lawyer Alan Shore on "Boston Legal," which ran from 2004 to 2008.
He guest starred on "The Office" last season as one of the candidates to replace Carell's Michael Scott as regional manager of Dunder Mifflin.
"In the finale, he kicked ass," a show source told EW.com about Spader. "He was so funny and had this weird energy. We didn't want to pass it up."
The Americanized version of the British workplace comedy is going into its eighth season.