Thread:Extension talk:WPMW/Importing old MW users into WP?/reply

For existing MW accounts, there is currently no way to automatically migrate. They would have to sign up for a WordPress account with the exact same name, at which point they would have access to their old MW account as well. They wouldn't have to use the same password, because once the integration is in place, the MW password is never used. NOTE: this means anyone could 'take over' an old MW account without knowing its password, simply by signing up with the same name in WordPress.

I think it would be possible to write a script that migrated old MW user accounts into WordPress - I haven't looked into the details, but if the passwords are hashed in the MW database (I assume they are) then it would also need to generate and email new passwords to the users so they could access the account.