Add one more thesp to the “Wedding Crashers” reunion on “The Internship”: Will Ferrell is set to join former co-stars Vince Vaughn and Owen Wilson in Twentieth Century Fox and New Regency’s comedy, which is being directed by Shawn Levy.Vaughn and Wilson play old-school salesmen who, finding themselves suddenly unemployed and passed-by in the digital world, try to reinvent themselves by becoming interns at a major tech company. Ferrell has agreed to cameo as Wilson’s brother-in-law, who sells home electronics and has no problem making inappropriate comments about his attractive customers. Ferrell previously worked with Vaughn and Wilson on “Wedding Crashers” where he played their mentor, Chazz Reinhold, in an uncredited cameo. He also made a cameo in 2004′s “Starsky & Hutch,” which starred Wilson and Ben Stiller. Max Minghella, Rose Byrne and John Goodman co-star alongside Dylan O’Brien, JoAnna Garcia, Eric Andre, Josh Brener, Tiya Sircar and Tobit Raphael. Levy’s 21 Laps Entertainment banner will produce with scribe Vaughn’s Wild West Picture Show Prods. shingle. Ferrell, repped by CAA, Mosaic and attorney Deborah Klein, next stars in Jay Roach’s R-rated comedy “The Campaign,” which Warner Bros. opens Aug. 10.