Do you want to add them for all users, or specific ones? Generally speaking, there is no easy solution for this, the only way to automatically add a Calendar is via Full access permissions/automapping. You can "share" them with individual users, which will then receive an email from which they can "Add" the calendar, if that's an acceptable solution you can use the Add-MailboxFolderPermission cmdlet with the -SendNotificationToUser switch.
