I dont think the game generates migrants on-site unless it has no choice. Your adventurers, for example, are far from standard dwarves. You cant count them as normal civilians because they themselves were procedurally genned.
Best would be getting an almost dead civ with only a few sites and tracking pops
I'm pretty sure the game regularly generates new dwarves for migrants. There seem to be two categories of migrants. Historical migrants, with a random selection of skills and skill levels and a smattering of social skills, and generic migrants, which I suspect are generated on the spot, which have semilogical sets of skills, usually no social skills, ad within a particular skill, all "generated" migrants have the same skill level if they have the skill. For instance, most dwarves that have a melee weapon skill have competent in that skill, a lesser level (plain x in dwarf therapist's glyph display mode. Adequate, maybe?) in assorted generic combat skills, like fighter, dodger, and armor user, and no social skills. Marksdwarves seem to be more commonly "historical", since they are usually hunters, which is a fairly common profession in worldgen, and arrive with recorded kills, more random skill levels and types, and often some social skills. It seems to me, though I may be imagining it, that "historical" migrants make up a much much larger portion of the migrant pool in 40.x than before. For instance, I got an adequate lasher with one recorded dwarf kill the other day. I really want to know whathappened in that dwarf's past, I should hunt him down in legends mode, but I have
never gotten a lasher or other foreign weapon skill (aside from miner, of course) from migrants in any previous version since dwarf therapist was released. He also was only an adequate lasher. Previously, migrant melee weapon skill was competent or nothing. Now I'm gettig a smattering of both higher and lower weapon skills, almost always corresponding with presence of social skills and a more random distribution of other skills. But I still get a lot of migrants (seemingly more on low dwarf population worlds, though I may be imagining it) that are generic migrants, with predictable skill levels and no social skills.
TL;DR: I'm pretty sure I've been seeing "generated" migrants in every world, even ones with lots of surviving large dwarven civilizations. But I might be making up imaginary distinctions, I'm not quite sure...