Do you mean you want to simply add a button to create a new calendar entry from a button in an email? A simple @formula would do:
@Command([Compose]; ""; "Appointment");
If you wanted to script it, it would be this:
Dim ws As NotesUIWorkspace
Set ws = New NotesUIWorkspace
Call ws.ComposeDocument( "", "", "Appointment" )
Or were you looking to somehow link the email and the calendar entry?