It's really not a good idea to do so.   Upgrading in place is the best option by far.  If you have set your timeouts to 60 seconds and choose a quierter I/O period there should be no issue.  Being ready for an upgrade also means you should be ablel to handle a controller failure in case of an actual HW problem. First though.  You MUST upgrade the new member and all 6.0.x members to 7.0.9.   The new array will come with 7.0.x since that's the minimum revision for that controller.  Upgrade it to 7.0.9 as well.  Create a new pool for that member and don't set the RAID policy yet.  Do enable the 2nd iSCSI interface of course. What disks do you have in the existing arrays? Are you running VMware ESXi?
Hello, I have a group of 4 Equallogic arrays 1xPS4000 (Type: 70-0120) and 3xPS6100 (Type: 70-0400) These all have firmware v6.0.4 The new array is a PS6210XV. I am pretty sure that the firmware on this one will be newer probably with a v7.x release.  At the documentation for firmware upgrade it states that I should upgrade all the arrays on my group first and then add the new on the group. I was hoping to add the new array and move some of the volumes to that one, upgrade the one array that I emptied, add it again to the pool, empty an other etc etc. Is that possible? 
Add new member and update firmware