code: public void addItem(String text, T value, OptGroup optGroup) {
          if (!values.contains(value)) {
              values.add(value);
              optGroup.add(buildOption(text, value));
          }
      }
docs: Add item directly to combobox component with existing OptGroup
  
  @param text     - The text you want to labeled on the option item
  @param value    - The value you want to pass through in this option
  @param optGroup - Add directly this option into the existing group