You can provision a VHDX on your pre-existing NTFS volume (IIRC VHDX overhead is <=1% for Hyper-V, not sure if that's still the case and/or applies to DevDrive). But yes, I would say an additional drive is ideal. I did just that to put games on DevDrive.
This is due to the file system filters. It shows when using DevDrive where there are significant performance improvements.