Hi Karen

There doesn't appear to be an option to directly move a user from  booked to waitlist.

Users get added to the waitlist when the event capacity is exceeded and waitlist is enabled.

To add selected users to the waitlist as an admin 

• remove the users from the seminar and probably untick the notifications
• decrease the capacity to the number of people booked in the event
• Add the users you deleted - this should add them to the waitlist

Regards