It isn't an area of the Firewall which I know that well, as I tend to use web filtering solutions (such as Webwasher/Web Gateway) separately.
However, if you configured network objects for the hostnames or domain names you wished to whitelist and then placed these objects into a group you could then create a firewall rule to allow access to these objects without applying a SmartFilter application defense.
Place this rule immediately before your normal web access rule and this should allow the whitelisted sites to pass without being filtered and all other web access would skip this rule (as the destination won't match) and will then use the rule which does have a SmartFilter policy applied.
The other option (though this is just a theory) would be to create custom URL entries in the SmartFilter confiuration screen, add these URLs to one of the User-Defined SmartFilter categories and in your assigned SmartFilter policy set this category to "Allow".
thanks philm for your usual support