Well without using any mods that affect your class at this point in the game, it's (as far as I can tell) impossible to ADD a class with the savegame editor.
You can change the one you already have, but since there's only one class actually listed, I don't see any way to add or change your second class (the one you'd normally gain on Dantooine).
Soo.. makes it hard for me to test anything on that line. =/
My GUESS would be, if you DO use the Revan mod, and thus gain a second class on the Endar Spire, and THEN used the save editor, you'd be ok so long as you didn't try to change your STARTING class to anything other than scout, scoundrel, or soldier.
But as I don't use the Revan mod, I haven't specifically tested that. But then again, if you're already going to use the Revan mod anyway, why would you want to bother changing your class with the save editor?