add new reservations for registered users (as administrator)

Yes, you can add events in admin in the normal way, then change the event owner to the new username.

no, I mean can I add a reservation for someone else as eventowner?

Sorry, there should be an Add Bookings link in the Booking Stats panel on the Edit Event page for the event.

Yes i know, but can I do it (as admin) for other peoples? so they have to write themselves out if they do not wish to participate

Yes, you’d just add their details when you make the booking.