How do I re-add a user to a smart campaign?

Once a user has exited a campaign, there's no way to re-add them. You would need to duplicate the campaign to re-add them.

To stop users exiting your campaign, I would recommend including a "dummy message" in your campaign, so that they do not meet the "received all messages" exit rule.

A dummy message is a message that cannot be sent due to its audience rules - like Email is unknown, for example.