You can capture 3 types of walkers: normal, armored, and tank.

Normal walkers are automatically released into your outpost when it’s attacked. Armored and tank walkers you need to place yourself.

To place armored and tank walkers, go to your outpost and tap “Manage outpost”. Then choose one of the first two sections, as walkers cannot be placed in the third one.

The left side of your screen will show your unplaced walkers and number of spots left in that section.