I can't think of any way, if I understand what you'd like to do.
When you click on the New (message) button, you want someone's email address to show up automatically by default, right?
The best solution I can think of is for you to make a new entry in Address Book, an application in your /Applications folder using the email address and the person's name and particulars if you wish.
That way, when you create a new message, you can type the first one or two characters in the email address and Mail will autocomplete the rest.
For example, let's say the email address you want to use is firstname.lastname@example.org, and you have an Address Book entry for email@example.com. When you press the New (mail) button or press Command-N in mail, a new mail window will pop up. You can then type b (and possibly o) on the To: line and Mail will automatically complete the email addess.
Background: My alumnus provides a permanent email forwarding service, but not a mailbox. I configure the forwarding to my ISP account.
In the Accounts preferences pane on the Account Information tab for Mail:
Enter the desired "Reply-To" email address in the "Email Address:" field.
In my case, I enter my alumnus provided email address there and the username and password for my ISP account in the appropriate fields.
It won't appear in the Reply-To field when composing a message, but will be inserted unless you type something into the Reply-To field, which would override it.
P.S. Just tested this. My alumni account appears in the email headers as a "Return-Path:". If I reply to this message, it uses the alumni address for the To: value. If you need an explicit "Reply-To" header in the message, scripting may be your only other option.