The way I can think of is to peform migration from NTFS to centera, migration will recall CAB file to original location, extract and then migrate savesets to Centera.
How to use the NTFS to Centera Migration utility (NTFSCenteraMigrator.exe) http://www.symantec.com/docs/TECH49828
The other way is to create a new vault store and create Centera partition. Then perform move archive from source NTFS vault store to Destination Centera vault store. Once all archive successfully moved then delete original partition & vault store.
Enterprise Vault Move Archive Feature Overview http://www.symantec.com/docs/TECH129236
And you can also contact Symantec partner/Consultant to know if they have any better way.