Here’s how you can resubscribe hard bounced users.
Go to Users- > Add emails -> in the Manually add emails tab copy&paste the email addresses.
You can also click Clean emails to make sure the email addresses are valid.
Then under “Add attributes to these email?” select the attributes from the existing set (ex. Subscribers to all emails) and add the attribute Flagged as hard bounced as false as in the image below, and click Import users.