The DNS-325 does not support RAID resizing (
ShareCenter - RAID Resizing). Secondly the DNS-325 does not support hot swapping, so the device must be powered down to add/remove HDDs.
Your best option is to copy your data to another source or mount your existing RAID-1 array in another device; power down the DNS-325, format a new RAID-1 array with new HDDs; and copy the data back.