Let me try to explain. Consider an existing pool of 5 disks ( R5 4+1). The datastore created on this pool will utilize 4 disks at this time. Now, if you have some space left in this pool at the time of adding a new disk pack, any newly created storage element will make use of the new set of disks along with the existing. This is because it will stripe across all the disks available. The striping will continue across all the disks until your free space runs out on the first RG. After that, only the disks in the new RG will be used.  Again, if you add a disk pack to the existing pool, your existing storage elements that you created on the pool will not benefit from the new disk spindles. But if you are using thin luns, you might be able to utilize the new spindles as an when your storage area grows, because when your file system automatically extends, it will make use of the newly added disks to carve out the stripes.  If you want to make use of the newly added disks, you can create a new pool with the new disk pack, create a new lun and migrate the datastore to it. If you want to make use of all the disks, (old disks and the new disks) you can add the new disk pack to the existing pool, create a new lun and migrate the datastore to the new lun.  Hope that answers your question.
Recently a client asked for more IOPS from their existing lun (VNXe 3100). Yesterday we added an additional DAE to the VNXe. I assigned the additional disks to the existing storage pool and increased the size of the LUN to span the available amount of storage.  However, I have no way of confirming that additional IOPS are available on the LUN. Can this be verified somewhere in Unisphere?  I cannot not find any documentation concerning the behaviour of LUNs after disks have been added to an existing storage pool.
LUN performance after adding additional disks to storage pool