Well, I tested it out. Seems to work, I just duplicated the mesh pieces, and averaged normals, then combined mesh, then put it ontop the original pieces, transfered the normal info, and it worked.

BUT i cannot delete the copied mesh from which it got the normals, or move it even or the normals on the original change. So how do I break their connection while keeping the new normals? I tried freezing the normals, didnt work.