Yeah, for all kinds of reasons, I'd never recommend detaching databases/volumes from a LUN from a SAN and just adding it to a cluster, there's so many issues that can arise from this and I wouldn't even think about doing the same with system databases. As others have suggested, build a fresh cluster from the start with dedicated shared storage etc, then backup and restore each database in turn, that way you can test each one while still keeping the original database online, and you have a rollback position of the original server\databases available in case there are issues with the cluster during configuration and testing.