I think you’ll need at least one server connected to the internet which is running the Repository Mirroring Tool (RMT) to act as a proxy to the SUSE Customer Center (SCC).
Then you would need to register each of your dark servers to your RMT server which would then give them access to updates.
Alternatively I’m sure SUSE would sell you SUSE Multi-Linux Manager to achieve the same thing or there’s the upstream project Uyuni.