To add the Ranger Service to your cluster, use the availavle database information.
While setting up the service, use the database configured from the Ambari instance. Once Ranger is set up, it connects to the services available on the cluster. The connection creates a NEW Ranger repository for each of these services. Each repository contains default configurations. All the Service Repositories that were defined in Ranger from the Ambari managed cluster are visible. This contains all the custom policies.
To migrate the policies from the old repositories to the new repositories use a simple python REST API application. For more information consult the Cloudera account team for this utility.