Extend Memory Size and merging new drives into a Previous single drive

Hello there

We have NAS Dell Power Vault NX3200 where six external SATA hard drives are fixed and configured RAID 5 to a single Partition of 9 TB previous

Now we want to Increase the drive size, so we purchased another 6 *2TB drive and inserted into NAS of Empty Blocks

and my question is how to merge the newly purchased external drives and make online and upgrade as a same Single Drive without erasing any data

Thank in Advance