If you started the Q2A site before the Wordpress site it's difficult to then integrate using WP's login system. The only way is to move the users across to the WP users table, then let them all reset their passwords so their logins work with WP.
You could do it manually if there are not many or run a SQL query something like the following...
INSERT INTO wp_users (<fields>) SELECT <fields> FROM qa_users
...where <fields> is replaced by the appropriate fields in each table, e.g. 'username' in wp_users and 'handle' in qa_users.