config.yml

Injector:
  ManyManyList:
    class: ManyManyListWithHook

ManyManyListWithHook.php

<?php

class ManyManyListWithHook extends ManyManyList {

	/**
	 * @param mixed $item
	 * @param array|null $extraFields
	 */
	public function add($item, $extraFields = null) {
		parent::add($item, $extraFields);

		if($item instanceof DataObject) {
			$item->extend('onAfterManyManyAdd');
		}
	}

}