In mail.app, in the Account edit screen, you can enter multiple email addresses on the 'Email Address:' line. Separate each with a comma, and then save your changes. When you compose a new email from that account, you'll see a little pop-up for selecting which address you would like to use as the sender of the email. Very slick trick, and easier than creating an entirely new account.
You can't change the physical name, though - only the email address. If you need a different physical name, you'll have to set up different accounts.

