Yes, 3.6 does only supports greenfield for Netapp. If the array already had a storage-group or masking view, it has to either be ingested into ViPR or left as coexistance. With netapp either of these is not supported. So, it has to be greenfield which means a Initialized fresh box.
ViPR Controller 3.6 (3+2)  For other 3rd party Arrays (thru OpenStack Cinder), in this case NetApp block, it is my understanding that configuration supports "Greenfield" only. This configuration does not support Coexistence or Ingestion. Correct?  What that means to me is that the "greenfield" NetApp Block  storage array must not contain any volumes. Just ready for provisioning by ViPR Controller.   Correct?
Greenfield/Coexistence and Ingestion Support for Block