NIghthawk (R) X4s R7800 & VPN service

When you go into Advanced Tab you see VPN.  When you click on Vpn it only has PureVPN.  I was wanting to know if you can use another service? If so, How do I make it appear in the dropdown menu?  I am already signed up with another vpn service NordVPN and I would like to have it on my router.

thanks!