Hi,
well the easiest would be to use the api data export features for csv where you can customize your template and then request a csv string :
default values for the columsnheadertemplate etc.. can be found in the FS administration section Data Export screen.
DataExportTemplateCSVData csvTemplates = new DataExportTemplateCSVData();
DataExportTemplateCSVData.DataExportTemplateCSVRow csvTemplate = csvTemplates.
DataExportTemplateCSV.NewDataExportTemplateCSVRow();
csvTemplate.DataExportTemplateId = -1;
csvTemplate.CSVOutputTypeId = CSVExportOutput.SingleAnswers;
csvTemplate.ColumnsHeaderTemplate = ;
csvTemplate.ColumnHeaderOutputId = CSVExportColumnHeaderOutput.Text;
csvTemplate.AnswerSelectionMark = "1";
csvTemplate.AnswerUnselectedMark = "0";
csvTemplate.FieldDelimiterTypeId = (int)CSVExportFieldDelimiter.Comma;
csvTemplate.FieldDelimiter = "";
csvTemplate.TextDelimiter = "";
csvTemplate.ReplaceCRTypeId = (int)CSVExportCarriageReturnType.KeepCR;
csvTemplate.ReplaceCRCustomChar = "";
csvTemplate.SelectedAnswersOutputId = (int)CSVExportSelectionAnswersOutput.AnswerText;
csvTemplate.RespondentTextAnswersOutputId = (int)CSVExportRespondentTextOutput.AnswerText;
csvTemplate.ExportAnswersTypeId = int.Parse(ExportAnswerTypesDropDownList.SelectedValue);
csvTemplate.MultipleColumnAnswersTypeId = (int)CSVExportMultipleAnswers.Separated;
csvTemplate.MultipleColumnSeparator = "";
csvTemplates.DataExportTemplateCSV.AddDataExportTemplateCSVRow(csvTemplate);
string csvExport = null;
csvExport = new Export().ExportCSVSingleAnswers(csvTemplates, dataExportTemplate);
or if you want the other csv export mode use
csvExport = new Export().ExportIndivdualCSV(csvTemplates, dataExportTemplate);