Can I define the user id instead of having drupal automatically put it in during user create?

I need to modify the user_save function in user.module to read like this :

if (empty($account->uid)) {
	     // if we have a designated id, put that one in.
        if ($uww_uid){
		  $account->uid =$uww_uid; 
	}else{
		   $account->uid = db_next_id(db_query('SELECT MAX(uid) FROM {users}')->fetchField());
	 }  
  }

How can I do this outside of the core instead? thanks

I made a module which works with services with xml-rpc and allows you to assign id you wish to user register via xml-rpc you must send the field uww_uid in the xml . I implemented hook_form_alter and hook_user_presave . I have attached it here https://drupal.org/node/1851154#comment-6796536