I'll show you how you can add and read custom variables in WorkflowServiceImpl.startWorkflow() method.

Create a new QName and add this into properties parameter collection.

WorkflowserviceImpl.startWorkflow(....)
{

............
QName customVariable = QName.createQName("MyCustomVariable");
parameters.put(customVariable, "MyCustomVariableValue");


}


Reading the custom workflow variable during updateTask () method.
workflowServiceImpl.updateTask(...)
{
......


WorkflowInstance workflowInstance = workflowTask.getPath().getInstance();
String workflowId =workflowInstance.getId();
int colonIndex = workflowId.indexOf("$");
String executionId = workflowId.substring(colonIndex+1);
String MyCustomVariableValue=(String) getRuntimeService().getVariable(executionId, "MyCustomVariable");
}

Add the below private method to get activitiRuntimeService bean from bean factory.
private
RuntimeService getRuntimeService()
{
if (this.activitiRuntimeService == null)
{
this.activitiRuntimeService = beanFactory.getBean("activitiRuntimeService", RuntimeService.class);
}
return this.activitiRuntimeService;
}

Activiti Runtime service is used to get the custom workflow variable from a workflow instance.