Migrate Functionality

This is a question about migrating users to WP from a table of old user/pass, to new WP logins. Does this plugin check user/pass against WP logins and then the External database/table?

We want to migrate users over to stronger WP user/pass from an old system and this plugin looks like I can do that with the hooks over a period of time (e.g. 60 days). Locking them out until they use the newer WP user/pass.

Am I correct in that line of thinking? It certainly seems like that’s the case from all I’ve read. Thanks.