My actions:
1) Create a form
2) Add data
3) Add an extra field to the form
4) Add new data to the fields
What's wrong:
The data in the extra field - added in step 3 - won't export to the .csv.
The extra-field-data however are shown in the SubmittedFormReportField.
That's nice, but I need the csv-export. Does someone know a workaround for this problem?