First, hook into your existing form, find your Form ID.
/**
* Implements hook_form_FORM_ID_alter().
*/
function YOUR_MODULE_form_FORM_ID_alter(&$form, &$form_state) {
$form['#after_build'][] = 'YOUR_MODULE_form_FORM_ID_after_build';
}Replace MACHINE_NAME with your fieldset group machine name.