思路是：修改#submit为自定义函数，并在其中调用原_submit。

例子：

$sys_form['#base'] = 'system_community_settings_form';
$sys_form['#submit'] = array();
$sys_form['#submit']['nodewords_sync_columns'] = array();
...

function nodewords_sync_columns($form_id, &$form_values)
{
    $form_values['nodewords_keywords'] = $form_values['nodewords']['global']['keywords'];
    system_community_settings_form_submit($form_id, $form_values);
}