I’d suggest a much simpler way:
Install NethServer as usual from it’s ISO image to your planned / prepared VM.
After the installation, add in a second VM Disk, making sure it is on the right underneath hardware disk / raid you’re planning to use.
Format with XFS, as NethServer itself uses that. (Less issues with Backup, Disaster Recovery, etc…).
Then create a simple symlink!
If all fails, it’s simple to reinstall, and symlink the second VM disk…
NethServer Disaster Recovery will also work, you just need to check the symlink.
NethServer’s built in backup will also work, but I’d still suggest saving the whole VM using whatever you use on your Hypervisor - I am a Proxmox user…
My 2 cents