Implementation of critical hit damage multiplier for most rpgs is as follows:
Critical Hit Damage Multiplier = (Base Damage Bonus on Critical Hit + Critical Hit Bonus A + Critical Hit Bonus B + ... Critical Hit Bonus Z)
Some combat systems also allow for flat bonuses on critical hit. I have not seen any flat bonuses to critical hit damage in KoA, only percentage based bonuses.
To answer your question more directly: Critical Hit Damage bonuses are additive with each other, but multiplied to the base damage done on hit. The example box shows how crit damage multipliers work in KoA. I can't tell you the base critical strike damage multiplier for sure off the top of my head, but I'm extremely confident that it is 200%, as this is the current standard in rpgs (though it could be 150%, this is the second most common value). To explain why crit damage is added and not multiplied, I'll use an example set of armor and weapons. For a set of seven items (helmet,chest,legs,boots,gloves,weapon1,weapon2) all having a +15% bonus to Critical Hit Damage:
Using additive bonuses you would deal 2.00*(1.15*7) times crit damage; 305% total damage.
Using multiplicative bonuses you would deal 2.00*(1.15^7) times crit damage; 532% total damage.
If the crit damage were 30% per piece instead, you would be dealing 1255% crit damage. It gets more ridiculous as you go up from there.
Multiplicative multipliers are generally rare in rpgs because of exponential scaling. You do see them in games, often in the form of temporary but powerful buffs. But these are usually multipliers to total damage done, not critical hit damage.
Edit: For clarification, the formula for damage done on a crit is simply:
[on crit: (Weapon or Ability Damage Dealt on Hit * Critical Hit Damage Multiplier)]