Kensai/Magic Weapons

[Balord]

I understand how a Kensai's Signature Weapon ability works on Masterwork weapons, but I'm a little confused on how it works with weapons that are already magical. For example, a 3th level kensai wishes to make a +2 longsword his signature weapon. Can he spend the appropriate xp price and add a +3 enhancement, making this a +5 weapon in his hands? or is he limited to making it a +3 weapon, allowed only to add a +1 enhancement? Does the Kensai's class level limit on enhancement to his signature weapon stack with the initial enhancement of the weapon or is it a limit to how much enhancement the weapon can have in his hands?
(I hope I worded this in a way that makes sense.)

 

[drunkmoogle]

In your example, the Kensai is limited to "creating" a +3 weapon, though he pays a lower experience cost; the difference between a +3 and a +2 weapon.