Is Joss Stone even old enough to be called a diva? We'll find out when the 16-year-old British R&B prodigy joins veteran rock chanteuses Deborah Harry and Cyndi Lauper at the seventh annual ''VH1 Divas'' concert on April 18. The three thrushes join a bill that already includes Patti LaBelle and Jessica Simpson. The show, broadcast live from the MGM Grand Garden Arena in Las Vegas, is a benefit for VH1's Save the Music Foundation, which supports music education in public schools. Expect additional divas and would-be divas to be added to the bill as the concert date approaches.