And how about local spool files?
No problem, this is just one more select method called nnspool, so you want this:
(add-to-list 'gnus-secondary-select-methods '(nnspool ""))
Or this if you don't want an NNTP Server as primary news source:
(setq gnus-select-method '(nnspool ""))
Gnus will look for the spool file in /usr/spool/news, if you want something different, change the line above to something like this:
(add-to-list 'gnus-secondary-select-methods '(nnspool "" (nnspool-directory "/usr/local/myspoolddir")))
This sets the spool directory for this server only. You might have to specify more stuff like the program used to post articles, see the Gnus manual on how to do this.