I think the Excel export is a canned feature and I do not think there is anyway to manipulate it.
Have you thought about exporting your data into a database? That is the standard practice I usually follow.
The model would be that your form data which you want to report on is saved into a table.
Then you create an Excel file which uses the table as the data source. You set up the spread sheet the way you want it and whenever it is opened it gets the latest data from your table.
This completly automates both sides of the process. Users use the form and submit it as usual, other users open their spreadsheets and see the latest data, without any interaction or extra steps in between.
Also then you can create SQL reports, which are webbased for even more cool reporting without the need for excel.