How would one go about doing this?
If there are enslaved dwarves in a goblin civilisation, they won't emigrate to my fortresses, right?
But how about dwarves in an elven/human civvie? Would they?
I think I recall wilderness dwarves do so. At least, I have gotten a few waves of migrants, so they come from somewhere, at least.
Obviously a lot of luck in world gen if you hope to get migrants in significant number.
But at least one wave of migrants is hardcoded, right? Do they just spawn out of thin air?
And if I start more fortresses to up the number of total dwarves, will they just migrate from fortress to fortress, mitigating any new dwarves spawning/arriving?
The first two waves of migrants are hard-coded, but any after that have to come from your civilization. So if your civ died, good luck with a multi-generational fort!
As for building more fortresses, It seems like the migrants would wander from fort to fort like you described, but I don't have any experience with that.