It took me while to find the right solution here as it’s not obvious.
Say you own email@example.com and use that as alias in your GMail account firstname.lastname@example.org. It’s easy to choose you bar.com address as the from address in the GMail Web Interface, but how to hell do you get this enabled in Apple Mail.
Just put in email@example.com,firstname.lastname@example.org into the E-Mail address field of GMail Account settings in Mail.
Next time you write a mail you’ll notice a drop-down menu for the From field in the editor.