I think this could relatively easily be cobbled together from the Caravan and Squad menus/graphics:
If an option is added to the Trade Depot to "Found an Expedition", which ques a menu like the "military squad" menu, allowing members of the fort to be chosen as the initial seven outpost members, (the "Squad Captain" position becoming the "Expedition Leader"). Once the seven are chosen, that ques a "Trade Caravan"-like menu where items from the fortress are "given" to the Expedition. The value of the Fortress is decreased in a similar manner to the Trade Caravans, and the weight limits for a wagon are calculated as well. No limits on how much can be given to the Expedition, just on how much can be carried by a wagon. Once all the items have been chosen for the Expedition to take, the weight limits are calculated, and however many multiples of the wagons are then cued as as Carpentry jobs at the Trade Depot
Which, incidentally, I wish were just called the "Depot" and was attached to a separate trading area painted in like a Hospital, designated a "Market", but that's for another suggestion
, which in turn makes the number of wagons required to haul all of the stuff at the usual 3 logs per wagon. Then there's a prompt that asks if you'd like the Expedition to embark and retire the current fort? (y/n). "Yes" pulls up the overland screen used in choosing where the Expedition should be going, which also uses a basic fill function to "red" out any place inaccessible to the current fortress. Players scroll the blinking X over to where they want their new Outpost to be within the non-red areas, and that same fill function should be able to plot the shortest possible path to that destination. Once the site (and site name) of the new Outpost is chosen, that begins the "pack-up" phase usually used for the Caravans, where they eventually move off the edge of the screen.
I see two options from the point at which the old fortress has been retired:
1) An "Adventure mode" Fast-travel screen which plots the movement of the entire expedition with the singular goal of getting them and their wagons to the new Outpost site. As in "Adventure mode", there would be the ability to "d" stop and enter adventure mode proper, except the "hero" would this time be an immobile wagon (the "head" wagon - all others would also be immobile, but not part of that entity) and the seven members of the expedition would function as "companions". Obviously, encounters might cause stress or even the death of members of the Expedition during the journey (corpses stored in the wagon), but so long as at least one of the Expedition is alive, the "The Expedition to <insert name here> has failed" end-screen. Wagons will never "d" stop inside buildings, nor on top of them, so I'd actually think that they'd only be able to "d" stop while on a road/path indicator while in an area with buildings. Once the Expedition finally gets to the required spot, then the game is cued to the beginning Fortress screen, only changing "from the Mountainhomes" to "from <insert originating Fortress name here>" and the new Fortress then can commence as usual in Fortress mode. Caveat: The wagons would most probably not move nearly as quickly as individual adventurers. Maybe taking x10 or even x20 as many time units per movement. Stores from the Wagons would need to be able to be eaten by members of the Expedition - the ability to deduct stores (and the addition of any loot gained along the way) from an Expedition would need to be implemented.
2) Just skip the "Adventurer Mode" bit and go directly to the new Fortress site after the usual 2 weeks travel time.