Hi Pieter,
sorry, I was not in the office during the last week, hence I only see this now… :(
In principle, you can simply move content from the community to the knowledge base like you would move content to another community category. So in a topic page, simply use the drop-down menu under “Topic” to select the desired destination category of your knowledge base.
Maybe that’s why there is no dedicated topic about it here, as the functionality is not different. But I get that it can be easily overlooked, so I might promote your question to our knowledge base for that. :)
And this is a tip which you might find handy: Instead of simply moving content to your knowledge base, you have two more options which might be interesting in connection to this:
Copying content
Instead of moving content out of the original category, you could also choose to copy it. This way the topic will be displayed in both locations at the same time. Be aware that only the opening post will be copied. This could be handy e.g. if a user has written a helpful tutorial which you want to highlight.
Promoting content
You can also choose to convert a discussion topic into an article - this way the content will give a richer experience (e.g. you can add an article label as well as a hero banner). It will also give the content creator a good feeling of appreciation when their content is promoted like this. Note that you can only convert discussions to an article. However you can also convert a question to a discussion, so practically all content can be made into an article.
Please let me know if this is clear, or if there are any questions remaining.