As far as the rules go, it's doable. But the player would need some sort of Refresh allocation to account for their access to an item who's Enchanted Item Slots are associated with an NPC (unless they can talk a player into it). Two item slots cost 0.5 Refresh, so the remaining 0.5 to externalize it to an NPC wouldn't be bad.
So while it works for game mechanics, I'm still not sure about the story reason why a Pure Mortal would be entrusted with a Warden's Sword, since it's the symbol of magical police and the intent behind it is to allow them to kill rogue spellcasters without using their own magic.