TV just can't seem to get enough of its favorite redhead, Lucille Ball. Even so, Susan Lacy, executive producer of PBS's "American masters" series, admitted at New York's Museum of Television & Radio on Thursday night that it took her nearly 15 years to convince her network to do a documentary on the great comedienne (1911-1989). At the screening of "Finding Lucy," a new 90-minute, warts-and-all biography set to air on Dec. 3, Lacy inferred that Ball just wasn't considered serious enough by network executives for inclusion in the series. But, as former CBS executive Michael Dann, a longtime associate of Ball's, pointed out on Thursday, Lucy is one of the handful of great stars who walked the planet in the last century, along with "Charlie Chaplin, Orson Welles and Barbra Streisand." He also confessed that when Lucy came to the network in the early '60s with a proposal to do "The Lucy Show," CBS did not welcome her with open arms. "With 'I Love Lucy,' " Dann said, "we'd found the recipe for the perfect chocolate cake. Why try risking something new with marshmallows?" As for what finally convinced the network to give the redhead the green light: "To be honest, we really didn't have anything else at the time."