There are several use cases for expanding the root disk with additional storage to avoid overwhelming it:

• web proxy
• containers
  • storing the container overlays
  • storing the container data / state

Partitioning and formatting can be out of scope for VyOS CLI, but automatically mounting after boot, early enough so that dependent services can rely on the storage being available should be part of the VyOS configuration.

The CLI could be something like:

set system storage $name description $text
set system storage $name mount $path
set system storage $name device $path

Perhaps a special option can be added for bind-mounts.