How to declaratively define Strongswan VPN connections for NetworkManager?

I have enabled the strongswan plugin for Network Manager via networking.networkmanager.enableStrongSwan.

I manually set up my work VPN using nm-applet, but obviously this won't come with me if I reinstall NixOS, so I'd like to set up the VPN using nix.

The problem is that networking.networkmanager doesn't seem to have any sort of vpn configuration system. How would I go about this?