If you want to add an ingredient that isn't coming from a normal order, it first needs to be added to the "supplier" you'd like to receive it from. Adding a "Local Supermarket" supplier will allow you to add items from there to your Inventory. Then, when you run to the store to get something, you can create a manual receiving entry. In the Historical Orders tab, there is a button on the top right labeled "Take Receiving" that lets you do so. It will create an order for that supplier and you are able to mark down what you purchased and transfer it into your inventory.
Hope that helps!