Matt Donnelly Senior Film Writer“Young Rock,” an upcoming NBC comedy about the formative years of Dwayne Johsnon, continues to round out its ensemble.Actor Lexie Duncan has been cast as a high-school sweetheart for the superstar during his teens, where she will play opposite Bradley Constant (the 15-year-old version of Johnson).She’s set to play Karen, a popular, charismatic, high-school girl with dreams of attending Barbizon beauty school.