public class CustomRenderer : SpreadExportRenderer
{
    public override bool ReplaceWorksheet(string sheetName)
    {
        bool result = false;
        if (string.IsNullOrWhiteSpace(sheetName))
        {
            this.AddWorksheet(sheetName);
            return result;
        }

        var workbookField = typeof(SpreadExportRenderer).GetField("workbook", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);

        Telerik.Windows.Documents.Spreadsheet.Model.Workbook workbook = workbookField.GetValue(this) as Workbook;
        int index = workbook.Worksheets.IndexOf(sheetName);
        if (index >= 0)
        {
            result = true;
            workbook.Worksheets[index].Cells[0, 10000].ClearValue();// cell range can be modified.
        }

        var worksheetField = typeof(SpreadExportRenderer).GetField("worksheet", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
        Worksheet worksheet = workbookField.GetValue(this) as Worksheet;

        if (result && index < workbook.Worksheets.Count)
        {
            // Sheet was not the last one.
                
            worksheet = workbook.Worksheets[index];
        }
        else
        {
            worksheet = workbook.Worksheets.Add();
        }
        worksheet.Name = sheetName;
        worksheetField.SetValue(this, worksheet);
        // this.worksheet.Name = sheetName;
        return result;
    }
}

private void radButton1_Click(object sender, EventArgs e)
{
    PivotGridSpreadExport exporter = new PivotGridSpreadExport(this.radPivotGrid1);
    CustomRenderer renderer = new CustomRenderer();

    exporter.ShowGridLines = false;
    exporter.ExportVisualSettings = true;
    exporter.ExportSelectionOnly = true;
    exporter.FileExportMode = FileExportMode.CreateOrOverrideSheet;
    exporter.RunExport("../../../PivotData", renderer, "PivotData");
}