Hi Kate,
You can create a new download format that can be used in most downloadable tables of data across your Moodle site by defining a new class whose name will be 'table_{formatname}_export_format' in lib/tablelib.php See the examples in there for the existing formats. Then you need to add the human readable name of your format, as it will be appear in the drop down at the bottom of the table, to lang/en/table.php. Like this :
$string['download{formatname}'] = 'String that will appear in drop down';
That's it, it is pretty straight forward.
Jamie