It is recommended that you read this article in its entirety before proceeding with the migration. 

If you're moving to MemberMouse from another membership solution, then your current members may already exist as WordPress users in the database. If this is the case, you can easily migrate these users into MemberMouse by using the Import Wizard. Just create an import file that contains the email addresses of the users to migrate and run the import wizard. Importing is done on a per membership level basis. Users will not be able to access the member pages until imported to MemberMouse.  Read this article to learn how to Import Members with the Import Wizard

The import wizard acts as an updater.  Any information already in WordPress will be used for the new member, so to import an existing user, all that is needed is an email address.  If you do enter a password into the import file for an existing user, it will be ignored.  

WARNING (MM 2.2.5 or below): Make sure that the email address(es) you have linked to your administrator account(s) and any additional employee accounts are not present in the import .csv file. If these are included, a member account will be created and admin/employee rights will be revoked. Therefore you won't be able to log into your WordPress admin area with that account anymore. 

NOTE: You may also want to consider whether you want to Hide the WordPress Admin Bar from New Members before the import, as it will only effect members added after the setting is enabled.