We don't have a way yet to make a new mesh item. For bodyshop-type reskins of existing meshes (that get their own catalog entry, not default replacements), you can use this: http://www.modthesims.info/showthread.php?t=348082 We're expecting new hair meshes... one of these days.
As for new non-CAS items, folks have apparently succeeded in making new ingredients and books. Again, only using existing meshes. And Peter Jones is working on an object cloning tool