There is no such hidden macro or script. The functionality is internal to Outlook, and its behavior cannot be modified. If you want to write your own code, you have a couple of choices:

1) Use the TaskItem.Copy and TaskItem.Move methods to copy a task into a calendar folder and then modify the Body property of the item returned by the Move method. This approach takes the least coding, but might not actually work well at all. I've never tried it, because I like the greater control that you get from ...

2) Create a new item in the Calendar folder and set all its properties using data from the task. The code sample at http://www.outlookcode.com/codedetail.aspx?id=645 shows how to create a task from an email message. Creating an appointment from a task would be structurally the same, but would, of course, use different properties.