You do not have to repack. Rename it from *.rar to *.pak and your set.
Items from NPC is server side.
Stats like Attack for weapons, Defense for armor is also server side.
Code:
gameserver\data\static_data\npcs\
gameserver\data\static_data\items\
So, editing the NPC items (STORE) is useless.
But extra stats like Tradable/Untradable is client side.