You could just make a linking module for phpnuke that re-directs people to your stand alone forums.
For this example I'll re-direct to Google.
Under modules, create a new folder called Google (folder name is the link name). Within the google folder, create index.php with the following code:
><?php
header("Location: http://www.google.com");
?>
Then just activate your new module and de-activate your old one. Then your new link will be displayed in your navigation menu.
Hope this helps.