You already have a Box whose Orientation is set to Vertical and you can easily add other elements like buttons and labels to it. So add a button below the menubar as following:
Gtk::Widget* pMenubar = m_refUIManager->get_widget("/MenuBar");
if(pMenubar)
m_Box.pack_start(*pMenubar, Gtk::PACK_SHRINK);
Gtk::Button* pButton = new Gtk::Button(Gtk::Stock::OK);
m_Box.pack_start(*pButton, Gtk::PACK_SHRINK);
show_all_children();
P.S: For a better visualization I'd recommend using Glade to see how it should look, what should go where, how packing is done. And then you can easily do the same in your C++ code.