Once the slots are there, the enchanted item is assumed to be maintained in some fashion by the character. It's up to the GM and the player to agree on what this means in terms of narrative and flavour.
One option which comes to mind is that, as part of this character's role in wielding such a weapon, he is also instructed on the specific techniques needed to maintain the weapon properly, even if he was not the one who forged it, much like a swordsman would know how to care for a sword by keeping the scabbard oiled and the blade sharp, even though he would not have been the one to craft it.