Save mail in multiple mbox files

[moocow]

I love gnus, but I am too stupid to understand it properly.
I have a hundreds of files in mbox format or partly mangled (enriched)
by a few years of using vm.
What I would like is
  * gnus reads from imap
  * I can store an article (mail message) in a file (mbox format)
    with an arbitrary name

Some observations..
I am having a hard time persuading the nnmbox backend not to use
~/mbox.
I would be content if I could
  * read from imap with gnus
  * save articles to a holding place (preferably mbox format)
  * use rmail to move them to final places later

This would allow me to visit these files with the nndoc backend and
read or reply.

Many thanks for any advice.