How far I got trying to follow the instructions on [http://sandervanderburg.blogspot.de/2014/07/managing-private-nix-packages-outside.html]. Works, but what doesnt work is referring to my custom package set from within configuration.nix (see configuration.nix line 14). Double hyphens in filenames represent the folder hierarchy.

configuration.nix

{ config, lib, pkgs, ... }:

{
  nix.nixPath = [
    "repo=/path/to/repo/default.nix"
    "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs" # Default
    "nixos-config=/etc/nixos/configuration.nix" # Default
    "/nix/var/nix/profiles/per-user/root/channels" # Default
  ];
  nixpkgs.config.packageOverrides = pkgs: {
    custom = import <repo> {};
  };
  environment.systemPackages = with pkgs; [
    custom.nvramtool # this is what doesn't work
    […]
  ];
  […]

repo--default.nix

{ system ? builtins.currentSystem }:

let
pkgs = import <nixpkgs> { inherit system; };
callPackage = pkgs.lib.callPackageWith (pkgs // self);
self = {
  nvramtool = callPackage ./pkgs/nvramtool { };
  […]
};
in
self

repo--pkgs--nvramtool--default.nix

{ stdenv, fetchgit, iasl, flex, bison }:

stdenv.mkDerivation rec {
  name = "nvramtool-${version}";
  […]