“Saturday Night Live” trouper Molly Shannon has joined the cast of Warner Bros.’ mobster comedy “Analyze This,” according to sources, joining stars Robert De Niro and Billy Crystal.Harold Ramis is directing the pic about a mob boss who undergoes psychiatric therapy. Screenplay is by Peter Tolan, Phoef Sutton and Ken Lonergan. Paula Weinstein and Jane Rosenthal have producing credits. Crystal is also credited as exec producer. Lensing is set to start this week in New York. Shannon’s bigscreen debut comes this summer with a starring role in Paramount’s “A Night at the Roxbury” opposite fellow- “SNL” comic actors Chris Kattan and Will Ferrell.Shannon is repped by Steven Levy at Innovative Artists and is managed by Brillstein-Grey Entertainment.