I want to add postgres RDS in PMM, please let me know if this is possible and help me with the steps to configure.
Yes, Its supported. Here is the documentation link - https://www.percona.com/doc/percona-monitoring-and-management/2.x/install/docker-setting-up.html#. Have pmm server docker up and running, and then use web interface to add rds details.
Here is the better documentation link for agentless rds monitoring with percona pmm- https://www.percona.com/doc/percona-monitoring-and-management/release-notes/1.16.0.html.