.Mac allows you to use numerous aliases with your .Mac email account, but offers no facility for creating a unique user name for each. They are seem to come from the user name of the main address.
Andreas Amann (Mail Scripts 
developer) has posted an AppleScript on macOSXHints that lets you add a specific user name for each alias.
Two things: Back up your Mail.app preferences (~/Library/Preferences/com.apple.mail.plist) first and make sure that you have quit Mail.app before you run the script.
