If you have a single drive in the NAS now and you just want to double your capacity (No RAID), simply add the second drive. The NAS screens will ask you to either format it as a RAID drive or a second drive.
As I understand it, if you pull your first drive and put in a new drive, yes, it will be called Volume_1. I don't know how the NAS will interpret installing the original drive after that, maybe it will call it Volume_2 but I suspect it will want to reform the drive. I'm sure someone here knows this answer because they did it.
-Joe