Here's a complete answer which builds on the idea of using a placeholder but uses mostly xml to add the submenu.
If you have a menu like so called main_menu.xml:
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item android:title="My Menu"
android:id="@+id/my_menu_item">
<!-- A empty SubMenu -->
<menu></menu>
</item>
</menu>
Create another menu sub_menu.xml which will be used in my_menu_item:
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item android:title="SubMenu One"
android:id="@+id/submenu_one" />
<item android:title="SubMenu Two"
android:id="@+id/submenu_two" />
<item android:title="SubMenu Three"
android:id="@+id/submenu_three" />
</menu>
In your onCreateOptionsMenu:
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate your main_menu into the menu
getMenuInflater().inflate(R.menu.main_menu, menu);
// Find the menuItem to add your SubMenu
MenuItem myMenuItem = menu.findItem(R.id.my_menu_item);
// Inflating the sub_menu menu this way, will add its menu items
// to the empty SubMenu you created in the xml
getMenuInflater().inflate(R.menu.sub_menu, myMenuItem.getSubMenu());
}
This solution is nice since the inflater handles most of the work.