Any Broadway producer worth his greasepaint will tell you there's nothing like a pop star to spice up your long-running show. Apparently the folks at Rent took that literally; they went and got themselves an actual Spice Girl -- Melanie Brown, the artist formerly known as Scary. ''I get to be a heroin addict. She's very feisty, very vulnerable,'' says the singer, who had been living in L.A. pursuing acting and writing an album in her kitchen. (''It's just something for me,'' she says.) Mel B. makes her Great White Way debut April 16; she's signed through September. Now, where did I put that ''Girl Power'' T-shirt?