nixpkgsFJ = builtins.fetchGit {
    # Descriptive name to make the store path easier to identify
    name = "florianjacob-nixpkgs";
    url = https://github.com/florianjacob/nixpkgs/;
    ref = "journalwatch";
    rev = "60b9e339059e72d6ac3469509c41dacca57b2979";
  };
  pkgsFJ = import nixpkgsFJ { config = {}; };
...
  nixpkgs.overlays = [
    (self: super: {
      python3 = super.python.override {
        packageOverrides = python-self: python-super: {
          journalwatch = pkgsFJ.python3Packages.journalwatch;
        };
      };
    })
  ];