Is "Adds # to # physical damage to attacks with this weapon per 3 player levels" mod from Poet's pen considered as an added local damage'? I doubt it because the weapon quality does not affect this mod. But I'm also uncertain due to the fact that your offhand will not benefit from the mod though it's not local. Can you enlighten me?
No. It can't be local because it can't be worked out on just the item - the value depends on the player holding it. Local modifiers apply entirely on the item, and thus are in effect even before you pick the item up.