Android studio NavigationDrawer sub items that change on runtime

I really need your help on editing my navigation drawer. I want to add sub items to it as shown here:

or here:

I have tried to search for similar questions on google but didn't find an answer. To me, its seems that possible solution may be to add a group of items after 3rd item and make this group invisible, but when the user clicks on "parent" it will become visible. However I am not sure how to insert group of items after specific index in menu and that may not be the best solution. So I am asking for experienced developers to help me find the best solution.

For your first approach, you can make a little edit. That SubItem is also an item an instead of putting everything in group you should put in an item. Try this example below:

    <item
        android:id="@+id/nav_category_electronics"
        android:icon="@drawable/ic_email_black_24dp"
        android:title="Electronics">

        <menu>
            <item
                android:id="@+id/nav_subcategory_laptops"
                android:icon="@drawable/laptop"
                android:title="Laptops" />

            <item
                android:id="@+id/nav_subcategory_tablet"
                android:icon="@drawable/tablet3"
                android:title="Tablets" />

            <item
                android:id="@+id/nav_category_phones"
                android:icon="@drawable/smartphone"
                android:title="Mobile Phones" />

            <item
                android:id="@+id/nav_subcategory_gadgets"
                android:icon="@drawable/ic_developer_mode_black_18dp"
                android:title="Gadgets" />
        </menu>
    </item>