You have to create a plugin for this. With in the init function of the plugin add the following code

$user = elgg_get_logged_in_user_entity();
if($user){
    $text = $user->name;
    $href = $user->getURL();
} else {
    $text = elgg_echo("welcome:guest:user");
    $href = elgg_get_site_url()."login/";
}    
elgg_register_menu_item('topbar', array(
    'name' => 'welcome',
    'href' => $href,
    'text' => $text,
));