If my character is participating in Carion Crown can I use the Sword Scion trait from Kingmaker? (In general. I know that it all ends up being GM discretion.)
It's a campaign trait so I'm not sure I'd allow it. We tend to stick with using the campaign traits that apply to the current campaign. the rest of the traits are fair game.
Traits from other campaigns are still mechanically appropriate. Try suggesting an alternate flavor name and reason for your character having the same trait.
Re-skin them, essentially. With GM's permission, of course.