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    I found a way to do this, I modified file modules/newbb/include/forumform.inc.php like this :
(about line 169) :
 else {          
                  // Otherwise, check previous subscribed status... 
                 $notification_handler =& xoops_gethandler('notification'); 
     
        //if (!empty($topic_id) && $notification_handler->isSubscribed('thread', $topic_id, 'new_post', $xoopsModule->getVar('mid'), $xoopsUser->getVar('uid'))) 
    //{              
              $notify = 1;          
       /*}          
      else {              
              $notify = 0;          
       }*/ 
}  
In this way the notification on new post is enabled by default if a user replies to a discussion or creates a new one.
I know it is kind of dirty hack and to do right things I should have add an option to user profile about notification behaviour but this is the quickest solution I found.