Party members are placed on the Hawk according to conditions set forth by the Ebon Hawk OnEnter. This is a giant script, that evaluates all sorts of conditions, including:
1. Is so-and-so in the party yet?
- If so, need to put them somewhere, update their local/global variables
2. How much of the game has gone by?
- At certain points, things are supposed to happen, like....
3. NPC interaction, banter, cut-scenes. Variable evaluation to play such n such.
Global variables are how scripts know what has happened in the game. Substituting a party NPC is much more involved than changing one or two scripts.... Perhaps reading about Recruiting an NPC
would be helpful, since it seems to address your main concern.
Search function, my friend, or just plain old try googling....