Just ensure new array is visible on ALL cluster nodes before adding the mirror.
After disks have been added to dg on the active node:
On all nodes, do:
vxdctl enable
vxdisk -o alldgs list
On active node, add mirror as per TechNote mentioned above. Wait for mirror to syncronize. Once completed, verify that SG can be switched to all cluster nodes.
If successful, follow rest of TechNote to remove mirror and disks from dg.