You could always cancel the showing of the dialog, in the ShowDialog
event and show your own custom one. Or you could change the dialog completely as explained in this help article: http://www.telerik.com/help/silverlight/radscheduleview-features-customizing-dialog-windows.html
However, making the difference between the editing of the appointment and the creation might not be as straightforward. You can set a flag in the AppointmentEditing event and check it in the ShowDialog event and then set the appropriate dialog to show.
Hope this information helps.
the Telerik team