Add a new magazine slot to the grocery-store checkout line: the publisher of the National Enquirer will begin publishing a new magazine in January devoted exclusively to reality television. The editors of Reality Week expect a wealth of material. “Reality stars, you can’t keep them away,” says Richard Spencer, who will edit Reality Week along with OK. “It’s always ‘Come to my wedding!’ or ‘I just redid my baby’s room. Want to see it?’ ” The new magazine comes even as the overall market for celebrity gossip magazines weakens. Simon Cowell, for one, expects it to succeed: “Why hasn’t anyone done this before?” he remembers thinking when first told about the idea.