You can create a proxy VM as usual, test it for everyting working and then you can convert Advanced settings of that Qube. Convert it to template for disposable VM.
qvm-prefs NordVPN template_for_dispvms True
Now set it as default disposable VM.
qubes-prefs default_dispvm NordVPN
Now create a static disposable named VM from it.
qvm-create -C DispVM -l green Nordstatic
qvm-features Nordstatic appmenus-dispvm ''
Set Nordstatic as your gateway for all VM that should pass through VPN.
Set firewall rules for this VM (Nordstatic) so that it can connect only to desired VPN IPs.
Don’t forget to set your previous default dispVM template as default after doing all this.