There are few ways to manage this.
Program those variables as pass-in field and let client add the corresponding data in csv file once they download file
Tricky Way 1 (Pro User):
Prepare an external utility where client can store this confidential information against a unique id. When survey with same id is taken , data will get appended with main survey.
Tricky Way 2 (Super Pro User):
Prepare an external utility which accesses survey database and let client upload the confidential information themselves in database
Do let us know if you need further help.