You're correct, they'll have to be setup and connected as separate units, then present the storage to your VC.
I have a spare VNXe 3100 I inherited from another site and now want to use it to increase my existing storage for VMWare.  What we have currently is a VNXe 3150.  I am assuming the only way to add this 3100 to the 3150  is to add it as a separate system.  In other words, give it an IP in the same subnet as the other iSCSI storage where it can be seen by VirtualCenter, configure it in Unisphere using the VMware Storage Wizard.  You can't connect two storage processors of different models together, correct? Just making sure before I forge ahead and get it into the rack.
Best method to add spare VNXe to existing storage