Pidgin

If you want to use pidgin with the OTR plugin, you will find out, that installing the plugin with nix-env -iA nixos.pidginotr does not work. The plugin doesn't appear in the plugin list. To use it, you need to make a packageOverride. There are two ways to do it.

User local change

To make a change that only affects the current user, do this:

1. Put this into ~/.config/nixpkgs/config.nix:

{
  packageOverrides = pkgs: rec {
   pidgin-with-plugins  = pkgs.pidgin-with-plugins.override {
        plugins = [ pkgs.pidginotr ];};
  };
}

2. Install pidgin with nix-env -iA nixos.pidgin-with-plugins.

System wide change

This solution fixes it for all users.

1. extend systemPackages with the pidgin-with-plugins software:

environment.systemPackages = with pkgs; [
  pidgin-with-plugins
];

2. afterwards you need to override the plugins directive with pidginotr like this:

nixpkgs.config = {
  allowUnfree = true;
  packageOverrides = pkgs: with pkgs; {
    pidgin-with-plugins = pkgs.pidgin-with-plugins.override {
      plugins = [ pidginotr ];
    };
  };
};

Note: In order to affect your system by your nix-language-specific changes you have to evaluate it, run (as root): nixos-rebuild switch