There are a couple of networking prerequisites you will need first before being able to perform a merge. Firstly you must use Nimble Connection Manager on hosts that connect to the array, which is available for Windows 2008/2012 or VMware (Enterprise or Enterprise Plus licensing only).
Once your hosts have this installed you can then proceed to switching your networking from manual to automatic, and switching on "enable rebalancing". This will start creating new iSCSI connections to your volumes for you, which is essential for when arrays are merged & data starts to rebalance across the systems.
If all the above criteria has been met then you can go ahead and merge the systems. There is downtime for the array that is being subsumed into the group whilst the configuration is pushed across (this is seconds rather than minutes or hours), however remapping volume information to the hosts may take slightly longer. With a little bit of planning this outage can be kept relatively small.
Please dont hesitate to get in touch with support or your local SE who would be happy to walk you through this process. Ultimately though, if Nimble Connection Manager cannot be used then scale-out is not possible.
It only takes a couple minutes to import all the merged arrays lun information into one. Then you will need to go the the hosts of the merged array and change the iSCSI discovery IPs. The entire process is documented here Nimble OS 2.0 Part 5: Merging Two Arrays into a Single Group - Smells Like Scaleout