Currently it is not possible to add or modify calendar events on an EWS calendar. KOrganizer shows "Rights: read-only". It seems that it hasn't been implemented yet (from ewscreatecalendarjob.cpp).
Hello, i have the same problem. We just migrated to Exchange and i look for an alternative to OWA on KDE. Unfortunately a read-only calendar is a dealbreaker for me. Is there any chance that this is fixed soon? Or is there a different option to use the EWS calendar in KDE? Can i help somehow?