The procmail config file is at /.procmailrc. If you don't already have a procmailrc file, just create one.
Make an entry for each mailing list that is to be gated to your local news server. Something like this will do the trick for most cases, but might fail for mail Bcc'd to the mailing list.
For example if your mailing list is 'firstname.lastname@example.org' and you want the newsgroup to be called 'lists.jokes' your entry would look like this:
:0 * TOjokes@netizen.com.au |/usr/local/bin/mail2news lists.jokes
Note there's no space between the 'TO' and the list address on the second line.
You need to put a blank line between the lines for each group. Put as many entries as you want, using the same 3 lines for each group, but changing the list address and newsgroup.