From ErfWiki

Speculation

Instead of croaking an enemy, a unit can attempt to capture it. This is generally used for casters, who are rare and valuable, and not warlords, whose duty and loyalty make them unlikely to join their captors. It is unknown if basic troops can be captured.

Known Captives

Wanda and Jack Snipe joined Stanley after he captured them. Jillian has been captured by Gobwin Knob forces many times, only to "escape" each time.

Charlie offered to capture Parson and later made Ansom agree to only capture, and not croak, him.