//Note: Call within QueuedTask.Run()
var mainReport = Project.Current.GetItems<ReportProjectItem>().FirstOrDefault(r => r.Name == "USAReports")?.GetReport();
if (mainReport == null) return;
//Add sub report
var vermontReportItem = Project.Current.GetItems<ReportProjectItem>().FirstOrDefault(r => r.Name == "Vermont");
if (vermontReportItem == null) return;
Report vermontReport = vermontReportItem.GetReport();
mainReport.AddSubReport(vermontReportItem, -1, true); // If -1, the subreport is added to the end of the report.