Well, then, you should be able to create a calendar everyone can reference
in a public folder.  Inside that calendar you can create sub-calendars if
you wish.  However, unless it can be done programmatically, I don't know how
you'd "seed" the main calendar or why you'd even need to do that.  Everyone
should be able simply to add the entries they want everyone to see into that
public calendar.  That's what we do here.  All our vacation or out-of-office
times that need to be shared with the remainder of the department we place
in the departmental calendar in the public folder.  Since it can be
displayed side-by-side in Outlook with our personal calendars, it's a simple
matter to drag-and-drop items between them.

Programmatic approaches can be discussed in
news://msnews.microsoft.com/microsoft.public.exchange.applications or
news://msnews.microsoft.com/microsoft.public.exchange.development