an approach - on aswering a post the person answering has the option of placing the question and answer on the knowledge base if the question is already on the KBG then the reply is posted to the KB but hidden till moderated. If question and answer is added to the KB then it is hidden untill moderated. Sounds fairly easy but in real world is a bit more complex but still workable.
Problems - there are multi questions on the forums already asking the same question in a slightly different way - needs a bit of fuzzy logic to sort these out rather then leaving it all upto a moderator. The KB needs a formal approach - categoried strictly. KB seperate from the forum they simply link together. I use a similar system on my trouble tickets for my clients - over time they end up simply going to the KB or the trouble ticket system pulls the answer from the KB and thus answers their trouble ticket.
To work out the fuzzy logic for the DNN forums one would need to have a dump of the subjects fields in the current forums in the first instance ( this helps to generate the rules for the same qustion in a slightly different format) then if you where going to say build a KB based on the current DNN forums a desktop application that could search on subject field across the current dnn forums would shorted the process somewhat.
Once the KB is built then when a new thread is posted the kb is searched on the entered subject text and a list of possible answers if any are diisplayed before the poster enters his post text.
Just some thoughts