You might be able to make use of the QWinWidget class that can be found in the Qt Windows Migration library. This is not part of the standard Qt distribution, so you will probably have to build it yourself. The library was written to help in solving exactly your problem - porting from a Windows application to Qt and hosting a native Windows HWND inside of a Qt widget.
Edit: I see from posts you have made on this same question in other forums that you are already using the Qt migration library, so you are already aware of QWinWidget.
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