Yes, this is a correct approach. Since NixOS/nixpkgs is reproducible, the only way to use a specific version of a package is to pin the associated version/commit of nixpkgs. This method is independent of whether you’re using Home Manager or not.

The simplest implementation is the one you proposed where a custom instance of pkgs is imported:

{ config, pkgs, ... }:

let
  pkgs_pnpm94 = import (pkgs.fetchFromGitHub {
    owner = "NixOS";
    repo = "nixpkgs";
    rev = "c3392ad349a5227f4a3464dce87bcc5046692fce";
    sha256 = "1klhr7mrfhrzcqfzngk268jspikbivkxg96x2wncjv1ik3zb8i75";
  }) {
    inherit (pkgs) system;
  };
in

{
  home.packages = [
    # Regular packages
    pkgs.curl

    # "custom" version of `pnpm`
    pkgs_pnpm94.pnpm
  ];
}

If your configuration is organized by a flake, another way of doing it is adding this specific version of nixpkgs as an input and create an overlay. Here’s an illustrative example:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs_pnpm94 = {
      url = "github:NixOS/nixpkgs/c3392ad349a5227f4a3464dce87bcc5046692fce";
    };
    home-manager = {
      url = "github:nix-community/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = { self, nixpkgs, nixpkgs_pnpm94, ... }:
  let
    pkgs_pnpm94 = import nixpkgs_pnpm94 { inherit system; };
    overlay = final: prev {
      pnpm = pkgs_pnpm94.pnpm;
    };
    pkgs = import nixpkgs { inherit system; overlays = [ overlay ]; };
  in
  {
    homeConfiguration.default = home-manager.lib.homeManagerConfiguration {
      modules = [ ./home.nix ];
      inherit pkgs system;
    };
  };
}

In this example:

• pkgs_pnpm94 imports the pinned version of nixpkgs for the specific commit.
• An overlay is used to replace the default pnpm package with the pinned version.
• home-manager then uses the modified pkgs with the overlay applied.