Ok, how do we add an existing RHQ server to the 2 Prod servers already in place?
Just install another server, pointing to the same backend DB as the other servers. Everything will be done automatically. By installing a server that talks to the same DB, that will tell the new server it is to add itself and it takes care of things.
Here's docs on the HA stuff: https://docs.jboss.org/author/display/RHQ/High+Availability
And yes, when you install new Storage Nodes, they must be clustered with all the others. The rhqctl installer should handle everything for you when you install a new storage node. I'm not an expert in the storage node stuff, any questions about that would have to be answered by others.