If you need to convert a non-inventory item to and inventory item, simply click on the item from your Item List.
From the pop-up screen, hover over the Actions Menu in the upper right-hand corner. Select 'Convert to Inventory Item' from the drop down.
This will automatically re-map the financial accounts related to the item. Select 'Save & Close' to save your changes.
Your item will now appear as an inventory item in your Item List.