After a bit of trouble getting foreman configured to allow local repositories i’m now struggling to work out how to add existing infrastructure nodes into Foreman, it seems like the only option is to create a host.
Another option is to let the hosts be created by the report of a configuration management solution like Puppet or Ansible. This will by default create an unmanaged host, but combined with the default hostgroup plugin it can create managed hosts which get assigned to a matching hostgroup based on facts.