Hi Lucie,
I am not sure if this is the shortest way to do this, but during excution when you see the context tab of parameters.ReportHeader variable of ModifyReportHeader sequence, you have all the data you need. Now as it's not available dirctly(it's a string which contains other report formatting stuff also), you have to put extra code in your application to extract the exact data you need and then open an excel file and write into it.
I am also attaching a picture of variable value during execution.
I hope this will temporarily solve your problem.
Regards,
Anuradha