If you need to send messages to a NNTP server, and you don't find the function, which is sufficient for this job, then use this.
$stream = fsockopen($server,$port, &$errno, &$errstr);
fwrite($stream, "POST\r\n");
fwrite($stream, "Newsgoups "$newsgroup."\n");
$fwrite ($stream, " What ever you need to write\n");
.........
fwrite($stream, "The rest of what you like to write\n");
fclose($stream);
There are ofcourse not so straight, but if you know which parameters your message needs, you can do it like this.
You can build up a message with imap_mail_compose ();
When you a complete message then U can write it to the server, and the server will take care of the time, date, Massage-ID and so on, but you have to know the sturcture of the header which you make, and if you like to write a multipart message, then you better get to know it in details.
In case that you know a better way, please inform me.
Good luck.