Currently, there is no way to support walk-ins in HackPortal.
Proposal
Add a check on the organizer side to see if a user has registered after a certain cutoff time.
This cutoff time will be determined by a super-admin on the Admin portal. This date should be changeable, with any registrations prior to the cutoff being updated to their appropriate status.
During check-in, a popup will display on the admin's device informing them that the hacker is a walk-in or registered after the deadline. Admins will then have the ability to confirm or deny check-in. A hacker can be checked in later if their entry was denied. This functionality is limited to check-in, and a hacker's status as late registration should not carry over to other scans.