The export to excel functionality is not customizable. For the form responses you can use the forms module API
to retrieve them as data source and pass them to .NET document export functionality or another more convenient way.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items