I've definitely not done this as I abandoned my phpnuke site and built a brand new one in WordPress, but I did experiment with how I would convert it and ended up deciding it was way too much work. I had all kinds of nuke modules, including forums, and some custom modules I wrote, etc.
However, if ALL you want to do is convert your news stories AND you don't have too many stories, I would look into modifying phpnuke's RSS feed to get it to output all of your posts instead of just the last 10. WordPress has a converter for RSS format.
The file is called backend.php and at the very beginning you'll see the SQL statements with "LIMIT 10". You can change that number to 999 or 9999 or however you high you need to go. The issue is that if you have too many posts, your webserver may time out before it gets through all the posts.
In that case, you'll also notice at the top of the code that you can add "?cat=xxx" to the RSS feed URL and it will dump only a single category. Hopefully breaking the file into pieces by category will get you under your server timeout limit.
Now for the harder part, you'll pretty much only get sid, title, a link that is to your old nuke site, and the hometext (summary). If you are good with PHP and look up the specs on RSS, you should be able to pretty easily add the date, full text of the post, etc.
Or if you are better at SQL than PHP and RSS, here is a link to the list of fields in the latest version of WordPress: Codex.
As of right now that is:
Field Type Null Key Default Extra
ID bigint(20) unsigned PRI & IND Pt4 auto_increment
post_author bigint(20) unsigned 0 FK->wp_users.ID
post_date datetime IND Pt3 0000-00-00 00:00:00
post_date_gmt datetime 0000-00-00 00:00:00
post_status varchar(20) IND PT2 publish
comment_status varchar(20) open
ping_status varchar(20) open
post_name varchar(200) IND
post_modified datetime 0000-00-00 00:00:00
post_modified_gmt datetime 0000-00-00 00:00:00
post_parent bigint(20) unsigned 0 FK->wp_posts.ID
menu_order int(11) 0
post_type varchar(20) IND Pt1 post
comment_count bigint(20) 0