You can manually register them after you've collected their data from your site. To be honest, the bare minimum we'd need is an email address and name for the event's registration.
They don't even have to technically become a full contact if you don't want them to, by not including an option for them to join a specific list, anyone who you're registering that isn't already a contact would only be able to be communicated with via emails built for the specific event campaign.
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