I don't see any reason why not. It should just see the two drives as one big master and the other as a slave. You might have to format the current drive and move everything to the new drives. I could be wrong...wait for someone to confirm or deny
bldengineer is correct, you can indeed have a RAID 0 array and then another single harddrive...I currently have 2 HDD's in a RAID 0 array, and another 2 harddrive as single drives.