The section describing the creation of a NetworkAttachmentDefinition by applying a YAML manifest should list the possibility of applying it to a specific Namespace since it is possible when done via a CNO object.

    https://docs.openshift.com/container-platform/4.17/networking/multiple_networks/primary_networks/about-primary-nwt-nad.html#nw-multus-create-network-apply_understanding-multiple-networks