Little "expand" icon is a button.

Everything is possible and not everywhere are standard ways.

Just define appropriate "arrow-down" icon and attach it into Action  
-button whichyou place into toolbar. That works easily if you want to have  
"[foobar-text v]" -style toolbar menubutton. However, I'm guessing you are  
wanting to have "[[some-icon]v]".

Such can be done approximately like this:

Ingredients are:
1) One standard toolbar A ImageIcon (assuming 24x24)
2) One dropdown arrow B ImageIcon (8x24)
3)
public Icon merge(Icon pA, Icon pB) {
   final Image a = ((ImageIcon)pA).getImage();
   final Image b = ((ImageIcon)pB).getImage();
   final int w1 = pA.getIconWidth();
   final int w2 = pB.getIconWidth();
   final int h = pA.getIconHeight() + pB.getIconHeight();
   final BufferedImage image = new BufferedImage(w1+w2, h);
   Graphics2D g2d = image.createGraphics();
   g2d.drawImage(0, 0, a);
   g2d.drawImage(w1, 0, b);
   return new ImageIcon(image);
}

To be honest, you actually need to also have special ToolBarMenuButton, to  
manage menu show/hide logic properly. I.e. some additional logic is needed  
to make it behave correctly (= in similar manner than menubar menus).

For example,
- Managing keyboard
- Cancelling menu (escape, mouseclick)
- Opening another menu while toolbar menubutton's menu is still open