If there's no culture, development level, and way of life preferences, for different national states and ethnic groups, and humans are just interchangeable units, sure. Just add as many immigrants as you want, problem solved.
Adding, say, to a country an additional 10%-20% of its current population in people from another culture, to be the younger and more fertile group, in an aging domestic population, would absolutely go without issue.
At least, if we also ignore that immigrant origin countries all see fertility drops, many projected to reach sub-fertility rates themselves soon, of course.