OK if you rebuild the array with all four disks, starting with a 1TB, you will have the same as I do on my NVX, 2.7TB (4 x 1TB x .93, less one drive for redundancy). So from the 1848 GB you will get to 2793GB.
You can backup the array to one of the 1TB drives. Then remove both 2TB drives and install the second 1TB. Perform a factory default to wipe the 1TB drive and establish a fresh array. This will not take long since there is only one drive in the NV+ so no synch process will take place (that's what takes the time). Then when everything is settled down, remove the first 1TB from your Mac Pro and install it as the second drive in the NV+. A disk wipe and resync will take place. Some 6 to 9 hours, you will have a redundant array. At this point you can take one of the 2TB drives and add it as drive 3 and it will be wiped, added to the array and another resync will take place. When that is finished you can then add the second 2TB drive as the fourth and final drive to the array.
This is a lot of work for to me, little gain. As I said, adding a third 2TB drive at this point will double your storage to 3696GB.
I just noticed one more reason not to try to use the 1TB drives. They are WD1001FALS according to your first post and I noticed they ARE NOT on the Hardware Compatibility List (HCL): http://www.readynas.com/?page_id=82
They may work and they may not. But as I said, they would make good backup drives when mounted in the Mac Pro.
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad