In QB you increase stock by entering a purchase of the item, with qty and total cost

that stocks the item purchase with that cost for that qty, FIFO means average cost per purchase, and the first purchase cost is posted to COGS when you sell one.

Insure that in company settings you turn on the items table and purchase orders. 

Then when you enter an expense or bill, about half way down the left side is a title "item Details" click that and a table opens where you can select the item, qty and item total cost for purchasing inventory