You should be able to do this using the Editor, select the Equipment tab then the Material Tab under the Equipment tab.
Create a New (Blank) of a material.
Since this is material is for armor, uncheck the "Apply to Weapons" checkbox.
Since Shields do not offer a Max Dex value, uncheck the "Apply to Shields" checkbox.
Is Chitin's intended to supplant metal, leather, cloth, none, or some of the indicated?
Is a Chitin armor automatically a Masterwork item? If yes, select the "Force Masterwork" checkbox.
Finally, open the Eval Scripts and add a new script in the Pre-Levels phase with a Priority of 100 and a index of 1.
~ We are a full suit of armor, since all pieces must be of the same
~ material to grant the bonus.
~ We have to wait until now to reduce penalties because our armor check
~ penalty won't have been set until now.
container.parent.field[arArcFail].value -= 10
container.parent.field[arMaxDex].value += 2
~ Only modify the armor check penalty by 2, because we force masterwork,
~ and that's adding another +1
container.parent.field[arArmorChk].value += 2
If you decide that Chitin armor is not automatically Masterwork then uncheck the "Force Masterwork" checkbox and lower the value to "+= 1" for both arMaxDex and arArmorChk values.
Hopefully this helps and is correct.