1- Create your new forum in your Admin CP and note it's forum id (do NOT APPLY any permissions to your new forum after you created it)
2- Find the forum you want to apply(inherit) its permissions, note its forum id too.
3- Put this script into your Admin dir and call it like this
http://yoururl/board/admin/forumper.php?fromid=X&toid=Y
Replace X with the forumid of your existing forum whose permissions will be inherited and Y with the forumid of your new forum.
The script will handle the rest..
I know it's crude, but who needs "attractive user interface" anyway, it works.. I recommend applying it to your test board before you use it and backup your db just in case..