There's a mention of stun damage in the damage and healing section of the rules, but I'm not sure how to actually *deal* that sort of damage. However, it works as you say (for an NPC); if they drop to 0, they're just out, not dead.
I can easily see a few optional moves, though.
"When you're pulling your punches, you deal -1 damage and it's stun damage."
"When you attack with a weapon wrapped in padding or another non-lethal weapon, deal stun damage."
Etc...
It seems quite reasonable. Also, punching is specifically mentioned as doing possibly 1 stun damage. Further, you could have the PCs wrestle and grapple people, then tie them / chain them to hold them. This could be done perhaps with a defy danger or else custom grappling rules.