Campagnolo type bosses are 5mm x 0.8mm thread; some Shimano bosses are 4.5mm x 0.75mm thread. I doubt you could retap a 4.75 x 0.75 boss to 5 x 0.8 without problems. Shimano did make a conversion kit to mount Campagnolo type levers ona Shimano "A-type" boss:


Source: Sutherland's 4th Edition