I am using local NNTP server (inn2) for mirroring several groups which are of interest for me. For this purpose I use suck to feed the messages into my local nntp server. This is half of what you need - because I think that your users would like to post articles and you would like to post them to the external server.
I think that you might find this document useful:
How to establish a suck feed...
This tells how to setup the suck feed and the posting of articles. Hope that it will help.