I'm given to understand that ZED created 3DOs using sectors you would create levels from. Is it possible to do the same with JkEdit by creating a level in the desired model shape and then export it through ZED?
Yes Sir! That's what I do. Keep in mind though that level geometry has the surface normals one way and 3do's have the surface normals the other way so the surfaces will actually be 'flipped' if you will. But all you do is go to File->Save As in JkEdit and save it as a .jkl. You can import the .jkl into ZED then export it to a 3do.