To populate the additional fields they should first be defined in the additional fields section of the settings. You can specify a name, which is used as the name of the XML element when displaying the results, as well as when using the DataLoad API.
Once the field has been defined it can be populated either via the DataLoad
API or through the Data From Field section. The fields are positionally
numbered, and you can load Extra Field 1, 2 and/or 3 from the page that
is read. If you are loading from a META field you will typically want a
search of .+
and the Meta Field you are loading from.