I think what you want to do is champher the object. Bryce doesn't do that. The above methods work, but are limited and you might end up putting a cube boolean on the sphere to flatten the top. To get a true champher, you'll need a decent modeling program to make your objects then bring them into Bryce. One effect would be to check the fuzzy box in the materials editor, but I dont' think that's what you're looking for.