You can add it like this:
QWidget * mywid = new QWidget(this);
QPushButton * button1 = new QPushButton("Button1",mywid);
QPushButton * button2 = new QPushButton("Button2",mywid);
QHBoxLayout * layout = new QHBoxLayout;
layout->addWidget(button1);
layout->addWidget(button2);
mywid->setLayout(layout);
ui->dockWidget->setWidget(mywid);