Just because China formally annexed new territory it wouldn’t mean the people in that territory would be free to move wherever they like. China has some pretty strict rules on internal migration. You have to get permission to move, and cities are filled with illegal migrants who are Chinese citizens but have to dodge La Migra just like illegal immigrants here in the United States. And with the same social problems–the illegal internal migrants can’t get legal housing, they have no job security, they’re vulnerable to exploitation since they’re committing a crime just by living in the wrong city.
So if for some godforsaken reason China annexed North Korea they still wouldn’t let the North Koreans leave North Korea. They’d need an internal passport to leave. The same border controls would remain in place, only there would be Chinese soldiers on both sides of the border.