Underground people (olm men, bat men, lizard men etc) have their own civilization with ethics, likes and whatnot, while the aboveground & some underground -persons (including non civ affiliated gorlaks) are in effect wild animals and have no scope of civilized personality with empty blank heads regarding standings on traditions etc. but have personal facets who define who they are like all living things do. Trying to find a group with no fixed address other than a region with how easily dwarves are prone to get lost wandering might sound like a very taxing thing to do when the surface area might be very large, or other complicated circumstances affecting the journey (losing the rough target you're looking for, distracted by onset of madness, targets no longer exist).
Reasonably if a diplomatic action can be taken, then the player should be able to designate any person to be appproached by a fortress representative scmoohzer, but to take their chances against a large predatory wild-person is kind of inviting trouble.
There are a few possibilities, here:
One is that they're on your map, and you just plain need a button to press to send someone out to talk to them to invite them to your tavern. (This would especially be useful with the underground tribes, giving you some way to deal with them besides theft and murder.)
Two is they they have been spotted on your map or by some patrol at some point in the past, so you know they're out there, and you need to mount an expedition to find them and use diplomacy with them.
Three is that you don't know what's out there, and you're just sending out a search party into the wilderness to see what they'll find. This is even more an "expedition", and would presumably be something equivalent to the world generation-level adventurers that go out and find and tame random animals. You'd basically just set up a group of dwarves to head out into a patch of wilderness nearby and see if they can find something interesting, with options to do things like bring back plant seeds we don't have, animals we don't have, and options on what to do with sentient animal-people, such as diplomacy or spreading rumors or the like.
Having them survive long enough to contemplate such a thing is a feat within itself, just one of those ardous long term things like a tree farm or having children/immature animals in the fortress that need 10+ years of attentive demanded gametime to mature to usability.
Just another reason I think migration needs to be toned WAY down (as in, cut by 98%...), as it means nobody appreciates children, migrants, or passage of time. A lot of fortresses are abandoned to FPS death within 5 game years.
Besides, if 60 migrants arrive on your first wave, they're a faceless mob. If 1-2 migrants arrive, they're distinct individuals you can get to know like your Starting Seven...
This request isn't "already in", the user isn't suggesting that animal people should be able to come as visitors and later petition to join, which can happen. They are suggesting that the fort be able to actively recruit/interact with the 'randos that just stand around in the wilderness waiting to get killed by other randos' and other intelligent creatures that wonder around then wonder off.
There's a part of this (especially what it leads off with) that isn't already in, but what it leads off with is, which can lead to someone glancing at this thread thinking it
is already in. (Reading the OP straight-up says that there is currently no way to get animalpeople into your fort at all, when there is.) Hence, it's a good idea to edit the OP for clarity to remove the parts that are, and emphasize that which is not.