Short answer
HubSpot's KB: you can remove an opt-out of all email that a user added manually. If the contact opted out themselves, you can't add the subscription back in bulk; resubscribe them from their record. Preferences are tied to the email address.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Get the contact's consent before resubscribing.
3. Related
Opting out a contact and resubscribing.