I'm not sure if it can be done. The <Attribute>???</Attribute> I do not know of a tag that would do what you suggest. There is a <Attribute>BattleAutoCastSpell</Attribute>
You also have
<Attribute>UnlockSpell</Attribute>
But I'm unsure whether a weapon can have these game modifiers.
So, why not just give that particular bow the game modifier of +3 hit points and +3 spell resistance instead. The only thing that this would not be doing is using the lifeshard bonus, but I'm not sure this would be something a weapon should utilize.