CBS’ “Supergirl” pilot has added “Super” veterans Dean Cain and Helen Slater to its cast in top-secret roles, Variety has confirmed. Slater starred as Kara, the titular heroine in 1984’s “Supergirl” movie (to be played in the CBS pilot by Melissa Benoist), while Cain portrayed the Man of Steel/Clark Kent in ABC’s “Lois & Clark:...