R510 - need more space

I have a PE R510 fully populated with 3TB disks (20+odd TB full capacity) as my backup server.

My backup runs to disk and is nearly filling up the space on a weekly basis.

I need to add more disk, but how can I do this with a Dell product AND get the new disk/disks to be presented to windows as the same logical disk partition (so that my backup server just sees one target)?