As I said in my first post, I don't know much about modding characters at all, but I'm going to learn.
Although what I do know is that to be able to do what you're asking, you'd have to create a new character slot for Zack, and I just don't think thats possible. I think you can use the Jenova save editor to add Sephiroth to your party, but adding Zack's a different story, because with the Zack mod he replaces Cloud as the main character.
You can't just swap the models and expect it to make sense, because to add Zack to the party he'd have to replace someone else so that the game didn't crash, and if you swapped him with say, Vincent, it would be Zack with all of Vincent's animation, dialogue and weapons etc.
I suppose to get Zack you could use Jenova to put Young Cloud into your party and then swap the models for Zack and young cloud IN THEORY. But any time there was dialogue the game (I assume) would crash.
