I'm glad you are finding the application usefull. I am just about to upload a new version so you may wish to wait for a few days ebfore starting any customistaions of your own.
To answer your question, it really depends on how you wish to populate the extra fields.
If they will be manually maintained via the UI, then you simply add them to the list, ensure they are in the same group as the other other fields and add them to the 'Site Listing' content type. The site data webpart will enumerate any fields in
that group so you'll be good to go.
If you wish to populate the fields as part of the scan job, then you would need to update the 'Microsoft.MCSUK.SPSiteDirectory2010.ScanJobHelperMethods.GetSiteBeingScannedProperties' method to add your values to the hashtable withe the key representing teh
fields internal (or static) name and the value being the value you wish to add to that field for each site lsiting.