Mount & Blade II: Bannerlord – How to Recruit Prisoners
Once you have captured several prisoners, you can check the Party menu to see the prisoners in your captivity. In the beginning, you cannot carry many soldiers, and hence you might not be able to recruit them as soldiers.
But once you have unlocked a huge space for soldiers, prisoners can be your main source for recruiting new soldiers. For this, all you have to do is to select a prisoner and then name the unit you want to put him in.
Sometimes there will be a small arrow mark with some prisoners and you might not be able to recruit them.
The reason is that those prisoners do not want to betray their previous allegiances and hence you will have to wait until they start believing in your cause. Once the arrow mark is removed, you may turn them into a soldier immediately.