I found the sucker punch (Target_SingleHandedAttack) skill in the Shared > SkillData > Skill_Target file on row SingleHandedAttack.
But I have been unable to find where and how it is specified as a skill when the offhand is free.
My goal is to modify Sucker Punch to also be available when equipping a shield in the offhand and no weapon on the main hand.
I was able to modify the shield stats to include sucker punch which is almost what I want. But a problem arises when you equip a weapon then a shield. You can have a shield+weapon and both shields up and sucker punch. If you equip the shield first then weapon, the sucker punch skill goes away.