Hi,
With the Business edition, you can edit the emails via the menu System>Emails.
There, in the "preload" of the email, you can add a bit of PHP to do that.
Now, I'm not sure what you mean exactly ? Do you want to send a copy of the copy to that email address ? Or you actually don't want to send the email to the normal user, only to the email address in the custom address field ?
Supposing it's the later, you can add such code at the end of the preload and it should do the trick:
$data->customer->user_email = $data->cart->billing_address->XXX;
where XXX is the column name of your custom address field.