While many icons are provided, from time to time you may want to add something that is not provided. First, consider that the icon may be found where you don't expect. Use the Icon Browser's category and sub-category filters to search in other places. If that fails to produce the icon you need, it may be time to simulate it. Go to the Structural icon set and select one of the generic shapes. These include a square, circle and triangle. Use generic shapes to build your own pseudo-icon. Change the description in Icon Properties, so the icon summary reflects the intended icon.
Note: Icons are assigned levels, so they automatically appear at relevant heights. Generic icons will always appear at base level. So this process will not work for simulating icons that appear on top of others.