I didn't think that the Romans went into population movements much - probably thought it a bit too expensive. They were more into absorbing local elites and just running the bureaucracies. There were the settlement of retired veterans in colonies, and the import of auxiliaries for border control, but mainly they seem to have used their borders to prevent population movements rather than encourage it. Immediately post-Roman is quite another matter.
The initial settlement of Britons in Armorica was said to be as the result of legions, taken from Britain to support one side in a civil war, never quite getting back home again. The stories (not quite history, not quite myth) don't seem to make clear the position of the womenfolk nor locals in this.
Presumably in Imperial societies it was quite normal for young men to be soaked up in the Army and disappear off. Only to be replaced by a new contingent. "Green cuffs have gone away, Doli, doli...."