How to add data in an additional row on a table in reportviewer? In C # MVC

For example, I have in reportviewer in a table that I show the following data, grouped by account:

But it turns out, that for each account (n accounts), above the row where the name of the account is n °, I want to add a row with a data of the previous balance of the account. For example:

I have the query to obtain the previous balance, but I do not know how to pass it in the yellow row to the corresponding account in reportviewer, I would like to know if anyone has any idea of how it could be done. I do not think it can be done with groups, or in any other way. I hope someone has some idea. So far, so I generate the report viewer:

      public FileContentResult GetFileContentResultParametro(ReportParameter[] parametro, string nombreReporte, dynamic query, string nombreDataSource, String format, String deviceInfo, String fileDownloadName)
    {
        LocalReport report = new LocalReport();
        report.ReportPath = HttpContext.Current.Server.MapPath("~/Reportes/" + nombreReporte);

        report.DataSources.Clear();
        ReportDataSource reportDataSource = new ReportDataSource();
        reportDataSource.Value = query;
        reportDataSource.Name = nombreDataSource_DataSet;
        report.DataSources.Add(reportDataSource);
        report.SetParameters(parametro);
        report.Refresh();


        String mimeType;
        String encoding;
        String filenameExtension;
        String[] streamIds;
        Warning[] warnings;

        FileContentResult fileContentResult = new FileContentResult(report.Render(format, deviceInfo, out mimeType, out encoding, out filenameExtension, out streamIds, out warnings), mimeType);
        fileContentResult.FileDownloadName = Path.ChangeExtension(fileDownloadName, filenameExtension);

        return fileContentResult;
     }

I hope I can find some solution. Greetings