I'm afraid there's not currently an easy way to do this.
But if you're comfortable with PHP programming, you can modify the HTML output using an advanced theme. The information itself can be retrieved using the following snippets of PHP code:
require_once QA_INCLUDE_DIR.'qa-db-admin.php';
global $qa_db; // required to get database resource handle
qa_get_option($qa_db, 'cache_qcount'); // returns number of questions
qa_get_option($qa_db, 'cache_acount'); // returns number of answers
qa_db_count_users($qa_db); // returns number of users