How to extend (alter) webform submit function to add more funcitionality?
Just append your function to the #submit element to add it to the list of handlers.

MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if($form_id == 'webform_client_form_1') {
    $form['#submit'][] = 'webform_client_form_submit';
  }
}