You should make your cage in your 3D package, it is by far the easiest way to do it. xnormals works fine if you can wrangle the nasty ui beast. I just wouldn't use zbrush for anything like that. Duplicate the finalized low poly, use a 'push' modifier equivalent in Maya (http://bit.ly/MkqRnb). Cover your high with the cage.…
your LP should match your HP topology as closely as possible, and your cage should be pushed out far enough to fully encapsulate the HP. It's a general rule of thumb (there are other rules and exceptions) to create hard edges wherever you have texture seams and vice versa. Since you always harden 90 degree corners and…