I currently have a multi-server setup with a couple web and dns servers, and then a standalone ipsconfig server doing email. All are running latest ispconfig 3.1. Is there a way to join the existing mail server into the multi server setup, bringing along all its mail domains, etc?
There is no way to directly attach a standalone server as the id's of clients, websites etc. in its database would collide with the ones of the multiserver system. What you can do is to use the ISPConfig migration tool to import the websites, mail accounts. etc. into the multiserver system, either into one of the existing nodes or you add a new server to the multiserver system first and then import the existing old server onto the new server that is already part of the multiserver setup.