I actually just did this exact same thing today (well, with a new 1.5TB drive that was added to 1TB drive already in the DNS-321). So long as you don't check the button to re-format the existing drive in a RAID setup (its not checked by default and there is only one question on the page so you can't miss it) when you add the second drive the DNS-321 will recognize it and ask if you want to format it - which you have to do of course.
Everything went fine for me, I just have another standard drive called Volume_2 now. Make sure the original drive is in the right side and put the new drive (or the one you're adding) in the left side. And as the above poster said, make a backup of the existing data first before you do anything