How do I make a subproject with Qt?

Yes, you can edit your main project (.pro) file to include your sub project's project file.

See here

Here is what I would do. Let's say I want the following folder hierarchy :

/MyWholeApp

will contain the files for the whole application.

/MyWholeApp/DummyDlg/

will contain the files for the standalone dialogbox which will be eventually part of the whole application.

I would develop the standalone dialog box and the related classes. I would create a Qt-project file which is going to be included. It will contain only the forms and files which will eventually be part of the whole application.

File DummyDlg.pri, in /MyWholeApp/DummyDlg/ :

# Input
FORMS += dummydlg.ui
HEADERS += dummydlg.h
SOURCES += dummydlg.cpp

The above example is very simple. You could add other classes if needed.

To develop the standalone dialog box, I would then create a Qt project file dedicated to this dialog :

File DummyDlg.pro, in /MyWholeApp/DummyDlg/ :

TEMPLATE = app
DEPENDPATH += .
INCLUDEPATH += .

include(DummyDlg.pri)

# Input
SOURCES += main.cpp

As you can see, this PRO file is including the PRI file created above, and is adding an additional file (main.cpp) which will contain the basic code for running the dialog box as a standalone :

#include <QApplication>
#include "dummydlg.h"

int main(int argc, char* argv[])
{
    QApplication MyApp(argc, argv);

    DummyDlg MyDlg;
    MyDlg.show();
    return MyApp.exec();
}

Then, to include this dialog box to the whole application you need to create a Qt-Project file :

file WholeApp.pro, in /MyWholeApp/ :

TEMPLATE = app
DEPENDPATH += . DummyDlg
INCLUDEPATH += . DummyDlg

include(DummyDlg/DummyDlg.pri)

# Input
FORMS += OtherDlg.ui
HEADERS += OtherDlg.h
SOURCES += OtherDlg.cpp WholeApp.cpp

Of course, the Qt-Project file above is very simplistic, but shows how I included the stand-alone dialog box.