betaboon/etc-nixos-configuration.nix Secret

## etc-nixos-configuration.nix
...
  nixpkgs.overlays = [ (import <nixpkgs-overlays/testoverlay)];
...

## etc-nixos-overlays-testoverlay-default.nix
self: super:

{
  testpackage = super.callPackage ./pkgs/testpackage.nix { };
}

## someexpression.nix
...
pkg = pkgs.testpackage;
...
	...
	nixpkgs.overlays = [ (import <nixpkgs-overlays/testoverlay)];
	...
	self: super:

	{
	testpackage = super.callPackage ./pkgs/testpackage.nix { };
	}