NEW YORK — The Atlantic Theater Company has added the world premiere of “Six Feet Under” scribe Kate Robin’s play “Anon.” to its current season, opening Jan. 31 at the Off Broadway company’s new second-stage theater, Atlantic Stage 2.Directed by Melissa Kievman, the play is an irreverent look at the overlap of sex, relationships and addiction as observed through the bond that forms between a young woman and the pet therapist she hires for her morose cat. Remy Auberjonois and Michelle Federer head the cast. Previews begin Jan. 24. The Atlantic has had a hit on its Chelsea mainstage this season with the new David Mamet adaptation of Harley Granville Barker’s “The Voysey Inheritance,” due to wrap its third extension March 25. Uptown, the Broadway transfer of the company’s first musical, “Spring Awakening,” opened to rave reviews in December and has been steadily building its audience.