class MyMemberAuthentcator extends MemberAuthenticator {
public static function get_login_form(Controller $controller) {
$label=singleton('Member')->fieldLabel(Member::get_unique_identifier_field());
            $fields = new FieldSet(
               new HiddenField("AuthenticationMethod", null, $this->authenticator_class, $this),
               //Regardless of what the unique identifer field is (usually 'Email'), it will be held in the 'Email' value, below:
               new TextField("Email", $label, Session::get('SessionForms.MemberLoginForm.Email'), null, $this),
               new PasswordField("Password", _t('Member.PASSWORD', 'Password'))
            );
            if(Security::$autologin_enabled) {
               $fields->push(new CheckboxField(
                  "Remember",
                  _t('Member.REMEMBERME', "Remember me next time?")
               ));
            }
//push your custom fields in here
$fields->push( ... )
return new MemberLoginForm($controller, 'MemberLoginForm',$fields);
}
}

Authenticator::unregister_authenticator('MemberAuthenticator');
Authenticator::register_authenticator('MyMemberAuthenticator');