There is no Qt API for embedding a Qt application into another Qt application. If you wish to use widgets from one Qt application in another, then you can use Qt’s QAxServer module to turn your application executable into an out-of-process ActiveX server that can run the control, see
http://doc.qt.nokia.com/4.6/qaxserver.html
The client application can then use Qt’s QAxContainer to embed the control. Note however that widgets can’t co-exist in 2 different applications. The QAxServer will create a new instance of the widget whenever it is queried, as widgets can’t be shared between applications.