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.