In the upcoming release of S Family Manager (v3.2), this will be fixed. S Family Manager will recognize volumes created by the Xencenter NetApp plugin. You won't be able to manage those volumes using S Family Manager however.
The main value of using the Xen plugin with an S550 is to have Xencenter control the creation of volumes and the provisioning of LUNs within those volumes according to Xen best practice. You will not need to use S Family Manager for those operations.
Sr. Product Manager
Device Management and Infrastructure Software