Xajax does not have such a feature. However, ids are supposed to be unique on a web page so assigning new values shouldn't be any more difficult than $response->assign($id,$attribute,$data);
The current xajax form field getFormValues() depends on field names and I always assign a field name and id using the same value. This makes it easier if you want to assign values in your response..
If you ever stop learning you may as well dig a hole, crawl in and pull the top over yourself.