Thanks to The Google GMail Loader (GML), this can be accomplished. You can Visit their homepage
or jump straight to the downloads page
Currently this application is only available for use in a Windows or *nix environment.
said by GML Homepage:
The application works by reading your existing mail files, and forwarding them to GMail. It does not delete the mail from your local computer. Email can be set to arrive at GMail in either your 'Inbox' or 'Sent Mail'.
GML is free and is licensed to you under the GNU General Public License. There is a pre-compiled windows version avaiable as well as a source only distribution for other operating systems. Please see the download page for more information.