Check out this thread! It answers the question!
http://moodle.org/mod/forum/discuss.php?d=5440
The answer is that you create your category of 20 questions, and then on that category click the button right below the "Import questions from file" button that is labeled "Create Multiple Questions."
This will ask you how many questions you want from the pool and let you add them to your quiz. They show up as Random Question (Categoy) in your quiz, where Category is the category you got the questions from.
Hope this works out - I know I'm excited about it.