You can join a civ if you can get a site leader to accept you as a hearthperson (or maybe entertainer). You can make people like you by giving them gifts, especially named gifts. You can carve bones into trinkets, name them, give them to some lord or lady, then ask them to make you a hearthperson.
where you can trick the game into having people to give you their position. That automatically makes you a member of that civ.
you could also just retire in a civ site that usually get you citizenship though at that point you also could just unretire another adventurer.
also I think the reason the outsiders can't do this is probably a way how the game handles civilizations as it seems like they are all splinter off the main one the adventurer came from and having no civ to start from would either error out or draw from nothing which would lead to a civ with no entity or ethics to draw from.
so as a bandaid outsiders just don't get to claim.
though I guess a good answer to this would be to party up with a different adventurer so that you don't end up getting lock out by character creation, sure sucks for solo play but uhh man multi-adv party runs are a life savior on scenarios like these since you could have someone from a civ just claim the site then hearthperson the outsider and or be able to retire the group regardless of starting point since one of them became the lord of the site.