Once a custom XML file is used within a scan, the custom products and patches defined within the custom XML file will be displayed in Patch View.
They will also be displayed when adding patches to a patch group.
Be careful though. Just because a custom product or patch is displayed in Patch View or in a patch group doesn't guarantee it is still being used in scans and deployments. It only indicates that the custom product or patch was at some point included in a scan or deployment. If you remove a custom XML file from the list of active custom XML files (see Specifying Which Custom XML Files to Use), the products and patches within that custom XML file will not be used in subsequent scans and deployments. A custom XML file must be active in order to be used.