In a short: a csv file is a text file and knows nothing about relationships...
If you would like to export data into Excel workbook to keep corresponding data under [
+
] sign, then you have to write custom macro (or program). Note, that [
+
] sign is used to group data. See:
Outline (group) data in a worksheet - Excel[
^]
In other words, you
can't achieve that by using standard SSMS functionality.