extension.php

<?php	
public function beforeCompile()
{
	$builder = $this->getContainerBuilder();
	$targetServiceDef = $builder->getDefinition($this->prefix('serviceName'));
	foreach ($builder->findByTag('myTag') as $name => $attrs) {
		$def = $builder->getDefinition($name);
		$targetServideDef->addSetup('addFoo', array($def));
	}

}