Ronnie DeSillers, the 7-year-old Florida boy who was pledged $400,000 in donations for a liver transplant, will be transferred from Miami to a Pittsburgh, Pa., hospital Tuesday for preliminary examinations, his mother said. Maria DeSillers said her son can live no more than six months without a new liver, but the process of finding a suitable donor organ can take as long as 18 months. The boy’s case came to national attention when President Reagan telephoned him and pledged a donation. Later, $4,000 that schoolmates had raised was stolen, and news of the theft prompted a benefit concert in Miami Beach and other fund drives. Surgery and hospital costs could reach $500,000.