You can accomplish this if you create a sub report for the list items and include it in your main report.
Then handle the SubreportProcessing event and set the datasource for the subreport. Sample code below.
ReportViewer1.LocalReport.SubreportProcessing += new SubreportProcessingEventHandler(SetSubDataSource);
public void SetSubDataSource(object sender, SubreportProcessingEventArgs e)
{
var mainSource = ((LocalReport) sender).DataSources["DataSet1"];
var subSource = ((List<test>)mainSource.Value).First().subclassitems;
e.DataSources.Add(new ReportDataSource("SubDataSet1", subSource));
}