Don't think you can convert a normal VDC to storage.
You will have to delete the current VDC and build a new storage type VDC and allocate the same interfaces.
More details on storage VDC on Nexus 7k for referencef you don't have this link.
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/nx-os/fcoe/configuration/guide/b_Cisco_NX-OS_FCoE_Configuration_Guide/storage_vdc_on_cisco_nexus_7000_series_switches.html
-Raj