$GLOBALS['spip_pipeline']['name_of_the_pipeline'] .= "|name_of_the_function";
// Example of adding into the "insert_head" pipeline:
$GLOBALS['spip_pipeline']['insert_head'] .= "|name_of_the_function";
 
function name_of_the_function($flux) {
	return $flux .= "This text will be appended";
}