There is currently no inbuilt functionality in Domino to do what you want.  It's either manual or you can create a script.  You could be lucky and there could be a project on OpenNTF which does what you want.

There is third party software called Power Tools that I find useful. You can run a report to show which databases are missing from one of the servers in the cluster, and then select the missing databases and have it create the replicas itself or submit the create replica request to AdminP for processing. 

The job to identify discrepancies between members of a cluster can be scheduled through an agent, although you would still need to view the results, and select the ones that should be added to the other member of the cluster. Information about the product can be found on the vendor's site: http://www.helpsoftwareinc.com/Products/PowerTools