I found the solution in other people code, this is what i am using to freeze only the header row of the worksheet[0]
protected void RadGridReport_ExcelMLGridExporting(object sender, GridExcelMLWorkBookCreatedEventArgs e)
{
e.WorkBook.Worksheets[0].WorksheetOptions.AllowFreezePanes = true;
e.WorkBook.Worksheets[0].WorksheetOptions.TopRowBottomPaneNumber = 1;
e.WorkBook.Worksheets[0].WorksheetOptions.SplitHorizontalOffset = 1;
e.WorkBook.Worksheets[0].WorksheetOptions.ActivePane = 2;
}