FlexGroups are for unstructured data.
A database, by definition, is structured data. Could you? maybe? should you? Probably not.
If your Database has a number of large files, FlexGroups will likely not get much. Just stick with DNFS. It works very well and will give the same or better effect: If set up properly, Oracle should have an multiple NFS paths to every node and every volume and the Oracle DNFS will do its own thing to better performance.
Even if your database is smaller files, I am not sure it would be a good idea to use DNFS and FlexGroups for the same data,