This might even already be implemented in the upcoming release, but it doesn't sound like it. Anyway, it'd be easy to add with that code in place:
In addition to retiring forts and founding forts, just having a button and interace option for "send an expedition" where you get to choose some number of dwarves from your current fort, and then it auto-retires your fort, kicks you to the embark screen, and THOSE exact dwarves then simply become your starting group for a new fort that you immediately start. I'd want to be able to choose any number of dwarves from 1 to however many I have. They are not supplemented by any outside dwarves. This is our expedition, not the mountainhome's.
I don't know about supplies. I guess you could just send whatever you want to bring with you to the trade depot or something, and have the option to bring that stuff in the wagon with you?
This would allow infinitely cooler community forts and stories and character continuity etc. for relatively minimal effort. I guess it would technically be possible by starting X many separate adventurer games, having them all walk there separately, and retire, then killing all the original founders of the new location, but for obvious reasons, that is less than elegant.
Edit: alternatively, give the option when you start a fort to choose the specific dwarves you want from your civilization's historical figures pool (not allowed to take royalty, unless they live in a previously player fort). Then proceed as normal. Same basic outcome, even less coding. Not quite as fun, but still very functional.