Currently, the repository sync is only available for Red Hat repos in the RHUI entitlement certificate. To manage the custom contents, you need to create a repository and upload the packages manually.

2.6. Creating a new custom repository (RPM content only)

  https://access.redhat.com/documentation/en-us/red_hat_update_infrastructure/4/html-single/configuring_and_managing_red_hat_update_infrastructure/index#proc_cmg-creating-new-custom-repo-rpm-content-only_assembly_cmg-managing-repositories

2.9. Uploading content from a remote web site (RPM content only)

  https://access.redhat.com/documentation/en-us/red_hat_update_infrastructure/4/html-single/configuring_and_managing_red_hat_update_infrastructure/index#proc_cmg-uploading-content-from-remote-web-site-rpm-only_assembly_cmg-managing-repositories



It would be nice to have the option to sync packages from an upstream repository directly, as Satellite does.