Which format do you use ?
For now it seems you are using CSV which allows only to import data. If you
want to use formatting you could use HTML (Excel is able to import HTML
tables), XML (either the old xml format or what is now the native format),
use automation to control Excel (if client side)) or use a third party
product such as Aspose.Cells...
Basically it seems you will have to redesign how data are exported...