so this person wants to Create a separate Timeline but in adventure mode, then try to retire them into a Fort that probably does not exist in the adv timeline.
instead of just retiring the fort, I guess they could just retire the adventurer first then start a new fort in the same civ and hope the adventurer pops up in the migrant wave.
You can build an adventurer site, then start a fortress on top of it. Adventurer and his gang will be hanging out on embark. Gotta make sure your adventurer comes from the same civ as the fortress or they'll be labeled 'hostile' though (meaning 'not hostile' in that weird unretire way).
When I tried this, I found my adventurers labeled hostile (though he was harmless) in all circumstances and variations of my attempts. I think it has to do with having to claim lordship over the site to be able to retire an adventurer safely. Correct me if I'm wrong, though.
the hostile thing is an leftover from when toady making it so folks who embark anywhere over towns end up being attacked by their citizens, this isn't the case for folks who retired or settled there but aren't of the same civ or population group, so far they are good guards for any wildlife and ok to have around the fort although hell will break loose if you end up attacking them.
back when I played fort mode(to test out the toady's version of retiring a site) alot you could avoid having an army by retiring and unretiring and using the local guests who didn't get the chance to leave the site as guards.
now this whole thread is made for the use of Not retiring so having 'hostile' camp adventurer is the one of the surefire ways for the person who made this thread to get adventurers in their fort outside of breaking down and making the fort first then adventurer second, or retiring the adventurer into the civ that you're going to make a fort from and hoping they pop up in a migrant wave.
so basically with out retiring the only process is Adv first then Fort second.
though with retiring a player could just skip the process of getting their adventurer sign up to the fort by making their background be of a player fort citizen so they have the benefit of starting from the fort, and the ability to retire in it and work there.
edit: ok went and check to see if the old dfhack means of retiring saves the fort intact vs the df method and uhh hmm it seems like the issue isn't retiring the site more so the un-retiring is buggy. and a work around would be to set up an adventurer at the site( best if they are citizens of the site in their backgrounds or you set one of the fort citizens up as an adventurer before hand), then switch to fort mode using dfhack's mode set oh probably should go in and toggle off the resident flag on them, then mass clear designations.