Enterprise-grade Raid allows you to expand a container if the system detects the additional space. However, more complex systems do this and it requires a dedicated hardware (Raid controller) and battery backed cache. I highly suspect that WD’s implementation of it does not allow this. Even if it did, the CPU would be seriously underpowered to do the job and will take days.
I would simply pick up an external USB3 drive, copy the data off and rebuild. I doubt there is a faster route IMHO.
