I believe the JNGP tools allow you to specify the rawcode when creating a new object (unit, spell, etc.), and maybe allow you to edit them for existing objects (I'm not sure, since I've never used JNGP).
What I have done is to open the map with an MPQ editor, and edit the .w3u in the archive with a hex editor, to change the rawcode. It's a little awkward, but doable.