It's a single mbox format file that is broken up into individual messages which are stored in directories by date. A script to feed them into an NNTP posting program. There are currently over 10,000 such messages.
Well OK, it is not the kind of data volume that you could not afford to store in two different native formats. (given today's size of disk drives etc)
There are difficulties.
The Mailman authentication data is stored as a Python 'pickle' (compressed dictionary file). There is a program to export a list of users, but it does not permit querying a single specific user, nor does it export passwords.
NNTP has a Perl hook for authentication. Someone would have to write a Python program that could extract the necessary per-user data from the Python dictionary, and be called from the Perl NNTP authenticator.
Again a case where it would be so convenient to have a central store of authentication info that could be queries by many different services... E.g. when one of the APIs would be LDAP (as someone already suggested), it would be really simple to write a Perl hook that does an LDAP query.
Across the Atlantic. The NNTP server is in England, the Mailman mailing list is in San Diego.
My suggestion was to run INN on the gw server that now runs mailman, and keep it local. You could run a couple of servers around the world, but of course each of them would be facing the authentication issue. There should not be yet another "register here for access" thing.
Rob