However, if there are email addresses in your opt-out import that currently exist as HubSpot contacts, while they will be automatically unsubscribed, they still exist as contacts and will count towards your billable contact total. To prevent these contacts from being included in your billable total, you can delete them and then re-import as an opt-out list. They will not exist as contacts in your portal, and only their email addresses will be marked as ineligible.
If your opt-out list needs to be imported from an external system on an on-going basis you could use an integration like PieSync to sync these email addresses to your HubSpot account.