The easiest way to do this is to edit the analysis to excel page in /path/to/moodle/mod/feedback/analysis_to_excel.php
Using http://50.16.56.195/moodle2/documentation/html/analysis__to__excel_8php.html to identify details in the following example:
$xls_formats->head1 = $workbook->add_format(array(
'bold'=>1,
'size'=>12));
Other than this, I am still playing with it myself.