First of all, the Calendar application is not a part of the API. So, you can not extend it in any other way except by inserting your custom menu. So IMO you have two options -
1. Add an ApplicationMenuItem for MENUITEM_CALENDAR , and create your own "New Appointment" screen.
OR
2. Add the ApplicationMenuItem to for MENUITEM_CALENDAR_EVENT screen. Once the New Appointment screen opens, the user will have to click on your menu item. In the run method of the menuitem, get the current active screen, get a handle to the notes field, delete it from the screen and then insert it at the location you want.
Code:
Screen s = UiApplication.getUiApplication().getActiveScreen();
Field notefield = s.getField(index);
s.delete(notefield);
s.insert(notefield, newindex);
As I said, its not neat. But it is certainly possible.
And you can not do these modifications without the user clicking on your menu item.