Basically, you just create an FAQ book within the Knowledge Base.

Start by creating a book page that will be the FAQ table of contents. Give it an appropriate page title (FAQ on topic) and page alias, but leave the Body field blank. Select the KB as the Parent.

Create a new book page for each Q&A, with the question as the page title and the answer in the body text. Select the FAQ ToC page as the Parent.

Over time, you may identify other posts on the site (especially in blogs and forums) that you may want to include in the FAQ. When you do, simply select the entry, choose the "outline" tab, designate the FAQ ToC page as the Parent. The blog or forum post will automatically be added to the FAQ.

This process is explained at http://drupal.org/node/23701.