If you want to add a new Mail.app account without using Apple's wizard, here's the trick...
Press and hold the Option key on the keyboard, then click the '+' button at the lower left of the Accounts preferences section. This will add a new blank account, and you can directly complete the empty fields on the right side of the preferences panel.
Enjoy!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050502011428531