Right now, it’s not possible for users to re-enter a campaign. What I’d recommend is to duplicate the campaign, which will also duplicate the first message in the campaign. From there, I’d delete the duplicated message, and move the original message from the first campaign to the duplicated campaign.
Your next steps will be to build out the campaign (before going live) with the additional messages you wanted to add. The reason for doing it this way is because the users who exited the first campaign will be able to enter the duplicated campaign. But they will not receive the first message in the campaign, (because they have already received it) and will instead receive your subsequent messages. This also means that new users who enter the campaign can receive the first message.
Let me know if all of that makes sense!