Yes, what I thought works just fine...
Go the the post you want to use as the basis of the fork. Click Reply With Quote.
Copy the BBcode for the post-quote from the reply edit box.
Now, go start a new thread. Paste that BB code, including the "quote=username;postnumber" in the brackets into the post edit box for the first post.
The link back to the original post in its original thread will still work. If you are feeling industrous, you can manually reconstruct the old "Forked from..." link that originally appeared, as the forking module just put together BBcode for you.