Hi William,
Dom0 has no network connection, so it isn't possible to connect
network storage to it.
When the Qubes backup process is run, it asks which VM should the
backup be sent to.
I have a VM called Backup in which I mount a shared folder from
my NAS device. The backup process works fine using this. And I
know that restores from the shared NAS folder work OK too, because
I test that occasionally.
Good luck,
Mike.