You can use the "Base Texture is Tangent-Space Normal Map" option in xNormal, or most full apps like Maya or Max should do it, don't think you can do it directly in Zbrush though.
If you want to transfer an existing normal map, I'd take Bal's advice. The closest zbrush would be able to do is to treat the normal map like a texture/polypaint and transfer it that way, which wouldn't compensate for the different in tangents/etc on the new mesh.
If you just mean using the geometry of one object to create normal maps for another object, that too wont work directly. What you can do is import the new mesh, subdivide it a few times until it has sufficient geometry, and project the details of the sculpt onto it, and then bake.
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Check http://www.polycount.com/forum/showthread.php?t=91331 maybe.
If you just mean using the geometry of one object to create normal maps for another object, that too wont work directly. What you can do is import the new mesh, subdivide it a few times until it has sufficient geometry, and project the details of the sculpt onto it, and then bake.