Example using an overlay on a nixpkgs package used in home-manager with a nixos flake.

flake.nix

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    home-manager.url = "github:nix-community/home-manager";
    home-manager.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs =
    { self
    , nixpkgs
    , home-manager
    , ...
    }:
    let
      lib = nixpkgs.lib;
    in
    {
      formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixpkgs-fmt;
      formatter.aarch64-linux = nixpkgs.legacyPackages.aarch64-linux.nixpkgs-fmt;

      nixosConfigurations = {
          pinixos = nixpkgs.lib.nixosSystem {
            system = "aarch64-linux";
            modules = [
              ({...}: {
                users.mutableUsers = false;
                users.users."demo".isNormalUser = true;
              })
              (home-manager.nixosModules.home-manager {
                home-manager.useGlobalPkgs = true;
                # UseGlobalPkgs means nixpkgs override declaration must happen outside of
                # home-manager, otherwise the override will not be evaluated.
                home-manager.useUserPackages = true;
                home-manager.users.demo = import ./user_demo.nix;
              })
              ({nixpkgs, ...}: {
                # See above comment about the overlay
                nixpkgs.overlays = [ (import ./overlays_mopidy.nix) ];
              })
            ];
          };
        };
    };
}

overlays_mopidy-dbus-exception.patch

From 8f1a91ba660786704dba964953ace077cbab902d Mon Sep 17 00:00:00 2001
From: Mark Greenwood <fatgerman@gmail.com>
Date: Sun, 9 Oct 2022 13:00:30 +0100
Subject: [PATCH] Fix dbus exception where algorithm 'plain' is not supported

---
 mopidy/config/keyring.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mopidy/config/keyring.py b/mopidy/config/keyring.py
index 5e1b04dd11..36b1a8acbd 100644
--- a/mopidy/config/keyring.py
+++ b/mopidy/config/keyring.py
@@ -41,7 +41,12 @@ def fetch():
         return []
 
     service = _service(bus)
-    session = service.OpenSession("plain", EMPTY_STRING)[1]
+    try:
+        session = service.OpenSession("plain", EMPTY_STRING)[1]
+    except dbus.exceptions.DBusException as e:
+        logger.debug("%s (%s)", FETCH_ERROR, e)
+        return []
+
     items, locked = service.SearchItems({"service": "mopidy"})
 
     if not locked and not items:

overlays_mopidy.nix

final: prev:

{
  /* Pay close attention to what's happening with mopidy in pkgs/top-level/all-packages.nix, as going
   * directly after overlaying on mopidy leads to a world of confusing pain.  Namely the "mopidy-with-extensions"
   * is being built with the patched mopidy, but the patched version doesn't really get used because it is pulled
   * in as a library dependency to mopidy-mpd, and ends up with the non-patched version in site-packages. */
  /* The mopidy packages are all declared in a custom scope to ensure consistency, as such the scope must be
   * overriden to update the scope dependencies.  So we start by overriding the scope.  overrideScope' expects a
   * function with two arguments, like the overlay, and not like the attrs overrides. */
  /* With scope, pay close attention to the name, the "'" in overrideScope' is not a typo. */
  mopidyPackages = prev.mopidyPackages.overrideScope' (mopidyFinal: mopidyPrev: {
    # Inside the scope override, override the mopidy package.
    mopidy = mopidyPrev.mopidy.overrideAttrs (oldAttrs:
      let
        # Patch has already been merged for the release that follows 3.3.0, so check
        # the version to allow this overlay to be gracefully removed without a hard
        # break as soon as the next release is made.
        withDbusPatch = (lib.versionOlder oldAttrs.version "3.3.1");
      in
      {
        patches = (oldAttrs.patches or [ ])
          ++ [ ./overlays_mopidy-dbus-exception.patch ];
      });
  });
}

user_demo.nix

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

{
  home.username = "demo";
  home.homeDirectory = "/home/demo";

  services.mopidy.enable = true;
  services.mopidy.extensionPackages = [ mopidy-mpd ];
  home.stateVersion = "22.11";
}