You should really fix this in your excel sheets. Putting that one specific value on every row in an extra column would be the easiest fix. You'd have to make no changes to the script.
An other solution is more complicated. You could create a new table with two fields: the file name of the excel as SourceFile and the specific single value in a Value field. Create that table first as a mapping table. Then when you load the data from the excel files in the loop you can use Applymap to add a column using the file name to look up the value in the mapping table.