Short answer: You can’t.
In fact, is there any email client where you can? In all email systems, the standard is that all the addressee fields TO/CC/BCC apply equally to the Subject and Body fields – with the exception that BCC-ed people don’t see other people you might have BCCed. That’s how emai worksl; there is no help for it.
Now, you could write a Notes/Domino application where you put pseudo TO/CC/BCC fields on a form and compose different messages when the “Send” button is pressed – but that would be another discussion and a more profound one.