config/users/all.nix

{ config, pkgs, lib, ... }:
let
  to-import    = name: value: ./. + ("/" + name);
  filter-users = key: val: val == "directory" && lib.pathExists "${./.}${key}/default.nix";
  imports      = lib.mapAttrsToList to-import (lib.filterAttrs filter-users (builtins.readDir ./.));
in
{
  # TODO: does not work?
  inherit imports;
}