Read/process mbox file in Gnus

[Skip Montanaro]

I recently switched to Manjaro from Ubuntu and decided to (finally) give
Gnus a try (VM no longer seems to be actively maintained and I'd prefer not
to install it from source different than everything else - which comes via
elpa/melpa). Up 'til now, I've used VM to do the mail processing for my
spam work related to mail.python.org. The workflow goes something like this:

   1. Assemble an mbox file of unsure messages on mail.python.org (mpo)
   2. Download it to my laptop
   3. Process it, saving some messages as spam, others as ham, and
   discarding the rest
   4. Upload the new spam and ham mbox files to mpo
   5. Retrain the system with the new messages

I have been using VM for step three. I'd like to do that with Gnus. Looking
through the manual, skimming Google search output and skimming an apropos
list of gnus commands I didn't see an obvious way to read a Unix mbox file.
I'm sure I must be missing something basic. Pointers appreciated.

Skip