Actually, I'd forgotten, I 'd already wiped all the dietpi stuff last night as I had no use for it.
A couple of things about installing DietPi in vbox. Depending what software you choose to install inside DieptPi, you probably can get away with 4 to 8GB for the vdi disk size. Make sure you enable EFI in the virtual machine setting ( System > motherboard). This is essential for it to work in bhyve using the UEFI boot method. You will drop to an EFI shell in Vobx when you start the DietPi VM. Just navigate to "Boot Maintenance Manager" then to "Boot from File" then <EFI>, <debian> and pick the shim efi file to boot.
Lastly, to avoid needing to fix EFI booting in bhyve, apply the same fix as you would in bhyve to the virtualbox DietPi VM. Use this command as root in a DietPi: grub-install --efi-directory=/boot/efi --boot-directory=/boot --removable
After setting up DietPi as you wish in Vbox, Close the VM and convert the vdi to a raw file using the VBoxManage clonehd command. Copy the raw file to FN. with scp/winscp for example, to a place of your choice.
Create zvol on FN of same size, or slightly larger, as the raw. You can use the GUI or CL to do that. The dd command is of the simple format:
dd if=<path to raw file> of=<path to zvol>
The raw file path will begin with /mnt<your pool name/ ...
The path to your zvol will begin with /dev/zvol/<your pool name>/ ......