You could get around the module limitation technically by first going to another module, spawning him/her there and THEN removing them from the party.
Handy if you want that party member added for just a map, say, cleaning a base.
So, for example;
Talking to the new party member.
Getting taken to a new module where to use him, which is a one-shot (say, end of Taris).
Spawn, remove, add new NPC as member, leaving the old member behind.
Add to the .dlg that when the stuff is done, the new is removed and he/she re-added and re-warp somewhere else.
Of course, that's just one example, more could be done with that...