[dnk.it]

meanwhile I've solved it by myself

maybe it should be useful for somebody else in the future :

$tabselected = '';
$tablinks = '';

if (PP_SCRIPT == 'PP_CHAT' )
{
    $vbulletin->options['selectednavtab']='pp_chat';
    $tabselected = ' class="selected"';
} 
$template_hook['navtab_end'] .= '<script language="JavaScript1.2">function openwindow()
{	mywindow=window.open("chatrooms/index.php","mywindow","location=0,status=0,menubar=0,resizable=1,width=650,height=500");
mywindow.moveTo(screen.width/2-300,screen.height/2-250);
}
</script><li'.$tabselected.'><a title="Chat" class="navtab" href="javascript: openwindow()">Chatroom</a>'.$tablinks.'</li>' ;