[Kromm]

Just to be official: The Boxing damage bonus applies to Boxing attacks made by rolling against Boxing. The Brawling damage bonus applies to Brawling attacks made by rolling against Brawling. The Karate damage bonus applies to Karate attacks made by rolling against Karate. Since you can only use one skill per attack, the answer is clear: You can't "stack" bonuses. Use only the bonus that applies to the skill you're using for the attack.