Both Operations and Inventory processes will allow you to add costs after they have been completed.
Here we have a Packaging. This Operation is unique in that it can have both an Operation Cost and an Inventory Cost. When the Operation/Inventory process has more than a single choice to apply a cost to (such as this, Multi Additions, Transfers, etc.) a dialog will appear.
Click on the drop down to see the possible choices to apply the ad-hoc cost.
Once you select a target, you will see the Add Cost Form.
Fill in the form as you see fit. Then check back on the cost in the Cost Admin Console. You will see the cost is linked directly to the selected operation.