If you go to Admin > Users and Permissions > Synchronize Users to Contacts
ie at path wp-admin/admin.php?page=CiviCRM&q=civicrm/admin/synchUser&reset=1 you will see the option "Synchronize Users to Contacts: CiviCRM will check each user record for a contact record. A new contact record will be created for each user where one does not already exist. Do you want to continue?"
You can use this to generate the WP user for each record. Then you should be able to send them out an email telling them how to use their email to 'request new password'
Note the last instruction is based on Drupal know how, not WP but hope it works for you.
Afaik the integration will only take across an email address for the user. Hence if you have used a WP plugin to keep name and address, or a Drupal user fields/profile, then you will need to either use sql query or Export that additional data from WP/Drupal and import based on matching with the civi contact record based on matching the email address