So, I did a history search and didn't find anything like this, so hopefully I'm not retreading ground that's been too well stomped. Hirelings would operate along very similar lines to mercenaries, but perform non-combat services for your fortress. Hirelings do NOT perform workshop tasks, but instead fulfill other useful roles within the fortress. Your dwarves are rarely okay with the presence of hirelings, seeing most of them as foreign workers being brought in to steal their jobs (even the jobs no self-respecting dwarf would even desire). However, hirelings and individual dwarves CAN develop friendships over time (though a dwarf can still at once be friends with a hireling and yet unhappy to have the group around).
Bands of hirelings could develop ‘relationships’ with your fortress. The more and better amenities they’re are given, the happier they will be and harder/faster they work, the less likely they’ll be to flee during a siege, and the more likely they’ll be to renew their contracts or later return to offer services again. Hirelings who are abused with substandard accommodations, broken contracts, late wages, wars waged against their own race, or theft of personal belongings by your dwarves will demand greater wages before they agree to serve you again or simply leave all together. If relations degrade enough, they might even rebel and begin rioting inside your fortress.
Hirelings would only appear once your economy has been activated (and possibly only after you have minted coinage). Each band of hirelings has a leader/captain, who will seek an audience with your outpost manager to discus the possibility of work. Hirelings are contracted to serve for a particular length of time (measured in either seasons or years). Contracts can be broken early, casting them out of your fortress, but this can lead to bad relations. Their wages are paid as any units are. During contract negotiations, you can offer to pay them more than they request, which makes them happier, or attempt to use your outpost manager’s negotiation skills to haggle them down, which makes them unhappy.
Bands of hirelings require a barracks to themselves (which would require the addition of being able to ‘(a)ssign’ barracks to specific groups of units, military or, in this instance, ‘other’), with enough beds, coffers, and cabinets for every member. Their leader will want his own room, though will sleep in the barracks if one is not given (albeit not happily). They prefer their own dining area, though will grudgingly use the communal one if none is provided. While not necessary, providing the members of the band with their own individual rooms will increase their happiness, as will giving them exceptional furnishings to admire. Hirelings will spend their wages at your shops, buying food and drink, clothing, and equipment (if possible, ‘upgrading’ to a better standard of gear when available).
Example Hirelings
Dwarven Engineers: Engineers are dwarves who are skilled in advanced construction and architecture. Masters at their craft, they prefer to sell their skills to the highest bidder rather than to tie themselves permanently to any one community. Engineers mine, smooth walls, construct bridges, roads, walls, floors, and fortifications, and will fulfill any construction project which requires an architect, mason, or carpenter. They do not engrave walls, leaving the task of charting the history of your fortress up to any native engravers you posses. Engineers tend to be more heavily armed than most other hirelings, possessing both sturdy picks and usually a hodgepodge of armor which they will occasionally spend earnings at your shops to improve. They’ll even fight, if cornered, and can prove to be an unexpected stumbling block to raiders and beastmen clamoring out of a chasm.
Human Roustabouts: Every fortress needs a few strong backs. Burly and cheerful, bands of roustabouts appear from time to time seeking work among their squat-legged cousins. Farmhands following the changing seasons or day laborers who’ve exhausted jobs in their own communities, these doughty humans handle any hauling labors your fortress needs. They also clean and help bring in harvests from your fields, though they do not do any planting. Roustabouts get along better with your dwarves than any other mercenary or hireling, even dwarven engineers (whom your native population tend to see as snobbish), and having a band of roustabouts in your fortress does not cause unhappy thoughts. They don’t even mind eating in the same dining hall, though providing them with one of their own will still make them happier.
Elven Foresters: Foresters are a “good will” group of druids sent out by elven nations to their less nature-friendly neighbors. Once you’ve hired a group of elven foresters, you can designate a ‘Woodlot’ from the zone menu. Woodlots must be located above ground and contain soil capable of growing trees or else they will not function. The foresters will begin to plant and cultivate saplings in the woodlot. Saplings tended to be a forester will grow twice as fast (taking roughly a year and a half, as opposed to three). Once fully grown, they’ll first spawn saplings of their own to the terrain around them and then the foresters will harvest the tree for wood, typically getting two logs per tree. Foresters don’t need accommodations inside your fortress, they will simply bed down in the woodlot and the larger it is, the happier they tend to be. Similarly, they never buy anything from your stores, subsiding off fruit from shrubs and water from streams or pools (if neither is available, they will drink from a well and only buy booze if even a well is unavailable). Wood gathered by foresters doesn't count against the tree-cutting demands of elven ambassadors and the foresters themselves will grow increasingly unhappy with every tree outside the lot you cut down. Foresters come fully armed with bows and arrows and wooden armor. However, they will not defend your fortress, only themselves and the woodlot, attacking any hostile non-elven creature that steps foot in it.
Gnomish Ratcatchers: Ratcatchers are mountain gnomes who are skilled in their own unique version of pest elimination. Their sole function in your fortress is the elimination of vermin, whom they ruthlessly hunts down and store in a leather sacks they carry everywhere. If given access to a supply of gnomeblight, they will even use it as an insecticide to eliminate the clouds of flies who tend to gather around your food stockpiles and workshops, though care is needed as any food left out in the open that gets sprayed by the gnomeblight will suffer a serious drop in quality and may even become uneatable. The gnomes themselves don’t worry about this. Unlike your other mercenaries and hirelings, Ratcatchers never buy food from your shops (though they DO buy booze in excess), but instead simply reach into their bags whenever they need a snack and are not above eating rotten food left in your refuse piles as well. For fairly obvious reasons, your dwarves will VERY much prefer any Ratcatchers in your fortress to have their own dining area.
Kobold Trapmasters: Dwarves and kobolds almost never see eye to eye, so it is a rare event when a band of Trapmasters offer their services to a fortress. Far smarter than your average kobold nuisance, they understand the workings of complicated machinery and that their skills are probably more valuable to other races than their own backwards people. Trapmasters can lay and load traps, connect other machines to leavers and floorplates, and generally fulfill any job that requires the use of mechanisms. A bit of a doomed breed, Trapmasters are not any better liked by their own people than they are your dwarves. Those who work for dwarves tend to get particularly targeted by ambush parties from kobold civilizations and it’s not unknown for kobold assassins to attempt to sneak into a fortress and eliminate the leaders of such bands.