In the Russian Civil war of 1918-21, the incorporation of prisoners was so standard operation that some people changed sides many times. They had to choose from being shot or dying from starvation in the camp, or maybe remaining alive by "voluntary" incorporation. Such mass incorporations happened practically after every battle.
In the American Civil War, both sides recruited from their POWs. Most entered US service and smaller number of Union POWs entered Confederate service. See
During WW2 you had:
Russian Liberation Army.
These soldiers fought for the Germans and consisted out of former POWs.
British Free Corps. These fought also on the side of the Germans and partialy consisted out of British POWs and volunteers.
National Committee for Free Germany. While technically not an army, but you may consider their efforts an attempt to ...
The question is extremely vague because the main concepts are too generic (conquering armies, conquered populations; and that covers the entire time span of war history up to the present); different moments involve very different scenarios: there are great differences between the ways in which such a phenomenon could have occurred is a tribal union, in the ...