Actually... You can create a fort then immediately retire it. Send your demi-god all stat/skills candy coated adventurer there and have him claim the site. Then retire your adventurer. When you unretire the fort, there he'll be. I believe any of your companions will also join the fort in question... someone made a post about getting a goblin lasher in their fort this way. Specifically he did as I said (create/immediately retire fort). He then pumped up an adventurer and gathered up some companions. He and his posse tracked down those 7 dorfs squatting around their wagon and preceeded to kill all of them to claim the site. Upon retiring the adventurer/unretiring the fort, there was his Urist McSchwarzenegger and his posse as citizens of the fort.
For a more peaceful solution, I believe you just have to gain some fame (do quests) and then approach the civilization in question (that your fort belongs to) and ask some big-wig for citizenship. I believe you can retire directly into the fort without killing everyone and claiming it first. I suppose you could just make sure your dorf adventurer is of the same civilization as the fort you are building (so he can already retire directly into your fort)... but I don't actually play adventure mode. This is second hand knowledge gleaned from other posters and the wiki, so take it with a grain of salt.