I'm pretty sure that Wordpress stores passwords securely, which means that the password itself can't be retrieved from the information in the database, only checked after it is entered.
This means you simply can't automate the transfer of passwords from WP's database to Q2A's.
The best that can be done in this sort of situation is to copy across the password information at the moment the user is logging in. You can check it against the WP database using WP's hashing scheme, and then if it matches, insert in into Q2A's database using Q2A's hashing scheme.
In practice this means intercepting requests to the login page (can be done via a process module) and doing the right thing. It also means the user can't delete their WP users table as long as some users aren't migrated across.