How to have multiple names in Apple Mail?
In ~/Library/Mail/V2/MailData/Accounts.plist : 


In Root > MailAccounts > Item # where # is the account to which you want to link the new aliases.
Create a new Array called EmailAliases
In this Array add a dictionary call Item # for each name you want 
In each dictionary you created add 2 string fields : name and alias. 
In name you put the name and in aliases you put the mail address. 
Restart Mail.app
Enjoy.



Here is a preview of this in the Xcode plist editor.