Changing from Individual to Group bookings

Hello,
I have an event that was created as having only individual registration but the client then wanted to allow people to bring a guest.

I can change the Registration Type and all future (group) registrations will have a screen to input the names etc.

However, people are contacting the client to ask for their registration to be updated.
It would be good if there was a (back-end only) function to change/add the new registrations.

Tickets have been issued and without a change, they can only be scanned once.
This will mean that checked-in figures will not correspond to actual attendees.
- This could be a security issue at some events.

Thanks!
-Graham-