Hello @WijckmansDries
Thanks for posting this in the Canvas Community!
I was able to play around with the appointment signups in the calendar and I also looked over the documentation on this.
I confirmed that you cannot fill in a student that did not make a reservation unfortunately. You do have the ability to message students who haven't signed up for a time slot. The student would need to sign up for the time slot on their end.
I did see that there was some discussion taking place in the community HERE. One of the points listed was to allow instructors the ability to add and remove course participants from the Appointment sign up.
Hopefully this helps!
-Colton.