It is possible to use ApachePOI to create Pivot Tables. I have added it into the repository
https://github.com/FourjsGenero/fgl_apache_poi, see the entry at the bottom of the README. So if you want to give your users an Excel file with a pre-populated pivot table you could investigate a solution using that. There is a little quirk in that I could not get the columns/row to sort without using a java class that was not in the ApachePOI download, refer to the readme for details of 3 lines you have to uncomment and the Java library you need to find if you want that.
If you are getting into volumes of data there is probably also improvements that could be made to the way the data sheet is populated in the example.
Alex pointed you at Tutorials&Samples->Reports->Order Report->DynamicPivotTable which creates a GRW report on the fly. That same front-end could potentially give you the parameters to generate Excel PivotTable using the above ApachePOI, as well you could use Dynamic Dialogs to generate a Genero form/dialog on the fly.