Generally speaking, the stock appointment and recurrence dialogs are not extensible, they are patterned after the same dialogs seen in MS Outlook. The reason for this is that there are just too many possibilities for us to even attempt to handle them all cleanly (out of all the many requests like this one, no two have ever been exactly the same). The recommended solution to problems like this is to create your own dialog, cancel the appropriate event (BeforeDisplayAppointmentRecurrenceDialog in this case), and display your custom dialog in its place.

Having said that, The RecurrenceDialog class is publicly exposed, and derives from System.Windows.Forms.Form, so you can theoretically modify certain things about it, although you would be doing so at your own risk since we cannot support such changes. For example, the times in the dropdown you mention here are presented as ValueListItems in an UltraComboEditor's Items collection, so you could clear that collection and repopulate it with different values.