It's additive, and it should be. I know it's confusing that some +% bonuses stack multiplicatively but here's the difference.
Values that are given as a %, and have a d100 rolled against, stack additively. Dodge and fortification come to mind immediately. Values that are given as a %, and are multiplied by a base value to give a result, stack multiplicatively. Healing amp works this way, and "spell damage +%" used to work this way. Now it's no longer a %, it's just "Spell power", and it's additive, for balance purposes.
Now the only question is if Armor-Piercing equipment values should stack like dodge or only the highest value should take effect.