How to permit one sub admin to change passwords of other certain users?
Thanks to arpitr and my coworker J., I finally did it with a form on a page which is accessible only to selected roles via hook_permission(). The desired array() of uid=&gt;name of that certain role to pass on as select list #option comes from db_select(), which definitely was the most difficult thing to get:

function _get_users_by_role($role_name) {
  $query = db_select('users', 'u')
    -&gt;fields('u', array('uid', 'name'));

  $query-&gt;innerJoin('users_roles', 'ur', 'u.uid = ur.uid');
  $query-&gt;innerJoin('role', 'r', 'ur.rid = r.rid');
  $query-&gt;condition('r.name', $role_name);

  $result = $query-&gt;execute()-&gt;fetchAllKeyed();

  return ($result) ? $result : array();
}


For those interested in the custom block approach arpitr proposed, check the accepted answer on How can I display a change password form on its own? for a even more detailed description.