I don't think that's how the weapon's supposed to work. You use gems or enchantments to get weapons without arcane abilities to do arcane damage. Blades of Mercy already have natural arcane qualities. You cannot enchant them with fire paper, bolt paper or empty phantasm shells either. Besides, when you put a non-physical gemstone in them, you'll lose out on that tasty A grade skill scaling.
BTW, that are soft/hard limits for stats?