[soraminazuki]

> NixOS doesn't have any simple way to configure that.

It's been a long time since I last used XMonad, but wouldn't the following configuration suffice?

    services.xserver = {
      enable = true;
      displayManager.sddm.enable = true;
      desktopManager.plasma5.enable = true;
      windowManager.xmonad.enable = true;
    };

Even if there isn't an official NixOS option to configure things the way you want, you can always put whatever configuration you like with the "environment.etc" option. For example, the following configuration would create a file in /etc/example/example.conf with the line "name=value".

    environment.etc."example/example.conf" = ''
      name=value
    '';