Adding an additional hard drive to a RAID array in order to increase the capacity is known as array expansion. One example of array expansion would be adding a fourth hard drive to a three-drive RAID 5 volume. Since RAID 5 uses block-level striping, adding a drive to an existing array requires pretty much all blocks to be moved either within a drive or from one drive to another. Not only does this take quite some time, it is also dangerous. For example, a forced shutdown due to a power failure could cause you to lose all your data.
So, even though adding a drive to an array without backing all the data up is possible, I would advice against it.
If you want , you can add the new hard disk as RAID0 or add another new one with it as RAID1.
Whatever you want to do , I advice you to BACKUP , BACKUP , BACKUP.