I tried the following:

Qt Code: 
  1. MyFileDialog::MyFileDialog()
  2. {
  3.  
  4.     QGridLayout* mainLayout = dynamic_cast<QGridLayout*>(layout());
  5.  
  6.     if ( ! mainLayout ) {
  7.         assert(0); // in case of future changes
  8.     } else {
  9.  
  10.         QHBoxLayout *hbl = new QHBoxLayout(this);
  11.  
  12.         // add some widgets
  13.         m_cb = new QCheckBox("My Checkbox");
  14.         hbl->addWidget(m_cb);
  15.  
  16.         int numRows = mainLayout->rowCount();
  17.  
  18.       // add the new layout to the bottom of mainLayout
  19.       // and span all columns
  20.       mainLayout->addLayout( hbl, numRows,0,1,-1);
  21.     }
  22. }
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however I'm getting the following error:

QLayout: Attempting to add QLayout "" to QFileDialog "QFileDialog", which already has a layout