1. save the attached file postfix.mbox

2. start emacs -q and eval the following code for a couple of times
(let ((buf (find-file-noselect "postfix.mbox")))
  (gnus-group-read-ephemeral-group "gnus-read-nndoc"
                                   `(nndoc "abc" (nndoc-address ,buf)))
  (kill-buffer buf))

3. In the group buffer type ^ to enter gnus-server-mode you can see
   something like:

,----[ Buffer *Server* ]
|      {nnfolder:archive} (opened)
|      {nndoc:abc-ephemeral+nndoc+abc-ephemeral:gnus-read-nndoc} (opened)
|      {nntp:news} (denied)
|      {nndoc:abc-ephemeral+nndoc+abc-ephemeral:gnus-read-nndoc} (opened)
`----