Plugin for user import or custom import function

Is it possible to made custom import function or change existing one?

We didn’t allow users themself unsubscribe from the lists, instead we (admins) check regulary which members should be removed from the list and uncheck them.

This is way too slow and i want to simplifie the process. One solution might be that users in lists are checked during import and unsubscribed from the list if the email address does not exists in import list.