Assets can be added manually, without receiving them through the fulfillment process. To add a new asset, navigate to Checkout > Assets. Select an asset type that you want to add the asset to. Click on the “Add Asset” button on the top right of the screen. A dialog with fields for the Asset Type will appear. All fields including custom fields created will be available to add values. 
Alternatively, an agent can add an Asset manually from the Asset itself.