no but you can inject Javascript via the Customer Portal Header to redirect the user to your Force.com Site.

https://help.salesforce.com/htviewhelpdoc?err=1&amp;id=customer_portal_enabling_features_settings.htm&amp;siteLang=en_US

Using Force.com Site for Customer Portal. Can Customer Portal Login page URL be hidden/deactivated? in lieu of the Site URL
How to get hover message when mouse goes on a button.

Thanks 
