Well, I am feeling stupid at the moment. The answer is very simple and sorry i wasted a space on the forum. But might be helpful to someone with similar requirement. The code is as simple as :
IBaseFactory testFactory = testSet.tsTestFactory().queryInterface(IBaseFactory.class);
testFactory.addItem(iTest);
where iTest is the object from test plan
ITestFactory sTestFactory = (QCConnection.testFactory()).queryInterface(ITestFactory.class);
ITest iTest = (sTestFactory.item(63514)).queryInterface(ITest.class);
I am sorry again for wastage of time and space!