Roswell star Delfino will play Fannie, the sister of Dwight Schrute (Rainn Wilson), TVLine reports.
Garrett (New Girl) has been cast as Dwight's nephew, with both characters set to debut in an episode of The Office's ninth season.
A spinoff from The Office - titled The Farm - is currently in development and would follow Dwight and his family as they take charge of a large family farm and bed-and-breakfast.
Delfino and Garrett would both appear opposite Wilson as series regulars if the project is picked up to series by NBC. Dwight's brother Jeb would also be a regular character on the show, but that role is yet to be cast.
The Office will undergo a cast overhaul this year, with BJ Novak and Mindy Kaling both departing the long-running comedy. Jake Lacy (Better With You) and Clark Duke (Kick-Ass) have signed up as new regular cast members.
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