Lenoxsoft currently has prospect data in another email-sending platform, and they want to migrate it...

Lenoxsoft currently has prospect data in another email-sending platform, and they want to migrate it over to Marketing Cloud Account Engagement. The system has a list of mailable prospects and a list of unmailable prospects who unsubscribed/opted-out that do NOT exist in Salesforce. The marketing team wants to make sure that they stay complaint with the permission-based Marketing Policy while maintaining their database of unsubscribed/opted out prospects when they migrate this data over to Marketing Cloud Account Engagement. How should the data be imported?