[admin@nixbox:~]$ cat /etc/nixos/useradd.nix

users.users.admin = {
     isNormalUser = true;
     extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
   };

users.users.giacinto = {
  isNormalUser = true;
  extraGroups = [ "wheel" ];
  password = "foobar";
};