Unless Mercenaries count, (because they almost amount to almost completely custom characters, especially when you can respec everything about them and change them compleatly, so it would make sense to put that feature in just so they could have late additions have a good time)
I would say no. :/ Sorry dude, it sounds like your 4th friend is relegated to the role of side kick. (they wont get the option to become the chosen god woken either)
It might be possible to use a save editor to change that flag on Fane if the mercenary thing doesn't work out, but i wouldn't recommend it, as if you mess up it would make your save unstable, and possibly unusable.
It might be a fun idea for Larian to allow you to change that flag ingame at some point just to make it more fun for late additions to the party.
