Verified answer

You can just get the items and make the weapon, but you won't have all of the bonuses that come with having the combo card (boosted PP earning with the weapon, and using the Heavy Attacks when you hold the button down). However, once you get the combo card, then you do get those bonuses.

U can get the items and make it, the combo cards are for when u unlock it by lvling up or by making it.