Expand Time Machine Volume

Does anyone know if you can expand a created Time Machine volume on your Drobo, particularly the 5c. The only option the dashboard gives me is disable (delete). I have 3x4tb HDD and only single disk failure setup, so my total capacity should be ~8tb. But my Time Machine volume is only 1.6tb. I’ve asked support if its possible to expand or alter the volume without deleting it open to be questioned about my purchase date and warranty standing.