EXAMPLE : https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/monitoring/zabbix-agent.nix#L45
-------
zabbix-agent.nix
--
{ config, pkgs, ... }:

{ services.zabbixAgent = {
  enable = true;
  extraPackages = "[ nettools zfs-user ]";
  server = "192.168.5.10";
  extraConfig = ''
    Hostname = ${config.networking.hostName}
    ServerActive = 192.168.5.10
  ''; };
}
--
OUTPUT : error: The option value `services.zabbixAgent.extraPackages' in `/etc/nixos/modules/zabbix-agent.nix' is not of type `list of packages'.