But the proprietary drivers are too, just add

    services.xserver.videoDrivers = [ "nvidia" ];

or

    services.xserver.videoDrivers = [ "ati_unfree" ];

to the system configuration and do a sudo nixos-build switch to switch into the new system with NVIDIA drivers.

(NixOS is pretty fantastic in this respect, in the installation live environment I just added

     boot.supportedFilesystems = [ "zfs" ];

to the configuration and switched to the new configuration. I could then do a root on ZFS install without any extra effort.)

https://nixos.org/nixos/manual/#idm140737316292128