You can change default account activation URL that is sent in email to the user.

<?php
function my_component_init(){
		ossn_add_hook('user', 'validation:email:url', 'my_component_hook');
}
function my_component_hook($hook, $type, $url, $params){
		//$param contains the $this user instance
		//you may use the $url that is actiavtion URL and warp it inside addtional URL or anything else.
		return 'new url';
}
ossn_register_callback('ossn', 'init', 'my_component_init');