If you try to say you want to create child work items, you can do that.
Option 1: Create a work  item template and create the work item with sub tasks using the template.
Option 2: Create a custom follow up action/participant on the work item save(server) operation. Start there: https://rsjazz.wordpress.com/2015/09/30/learning-to-fly-getting-started-with-the-rtc-java-apis/ the Extensions Workshop provides you with the general information.

To create work items in a participant:

		IWorkItem phaseWorkItem = fWorkItemServer.createWorkItem2(type);

		// Set th attributes required for save
		XMLString targetSummary = thisItem.getHTMLSummary();
		XMLString addition = XMLString.createFromPlainText(" [  some info ]");
		XMLString newSummary = targetSummary.concat(addition);
		phaseWorkItem.setHTMLSummary(XMLString.createFromXMLText(newSummary
				.getXMLText()));
		ICategoryHandle category = thisItem.getCategory();
		if (category != null) {
			phaseWorkItem.setCategory(thisItem.getCategory());
		}

		IIterationHandle iteration = thisItem.getTarget();
		if (iteration != null) {
			phaseWorkItem.setTarget(iteration);
		}
		if (null != phaseType) {
			phaseWorkItem.setValue(phaseAttribute,phaseLiteral.getIdentifier2() );
		}

		// IWorkflowInfo workflow =
		// fWorkItemCommon.findWorkflowInfo(phaseWorkItem, monitor);
		Set additionalParams = new HashSet();
		additionalParams.add("some info to prevent recursion");

		fWorkItemServer.saveWorkItem3(phaseWorkItem, null, null,
				additionalParams);

Once they are created, you can link them.