Expand RAID 5 Array on PR4100

I have a WD PR4100 My Cloud on firmware 2.31.183 with 4 x 6TB drives configured as RAID 5 (giving me ~18 TB of space) with ~2.36 TB free.

I need to expand the RAID volume and want to add 2 more 6TB drives in an external USB 3 enclosure. I initialized the disks as GPT by connecting them to a PC, and formatted as NTFS without assigning a drive letter.

When I attach the drive enclosure to the PR4100, I only see 1 disk under USB connections (not the 2 that are inserted) and no way to add them to the existing volume.

How do I go about increasing the storage capacity of the PR4100?