Hi Andrzej,
Thanks for your interest in Syncfusion products.
As of now we don’t have direct support to edit the SfSchedule in-built editor Style. As you have mentioned we can create the custom editor style by cancelling the SfSchedule in-built editor by using the event AppointmentEditorOpening.
We have already logged the report to improve the Customization Editor demo which support to create Recurrence appointments though the custom editor. We will improve it in any of our upcoming release and let you know once it will be achieved.
Regards,
Jeyasri M.