Unconciousness and Non-lethal

Not every fight is to the death, and certainly not every blow is fatal.
In D&D there's a system in play where a player cannot die until they reach a negative amount of hitpoints equal to their constitution score (so someone with 8 constitution dies a bit faster than the hunk with 24 con) ...Also, there's such a thing as "Non-lethal damage" usually done by unarmed strikes without the feat or with a -4 penalty, though there are "Merciful" blades that deal non-lethal damage with an additional 1d6 bonus to its damage.

I'm doubting a separate damage pool would be all that easy to make, so instead what about if the "Killing Blow" was dealt by a non-lethal combatant, it just knocks out instead of skewers and massacres people?