Just add a value to the enumeration button_set and catch that in the javascript file.
e.g. in the xml add
<enumerationValue key="custome_style">custom_style</enumerationValue>
then in the simplerichtexteditor.js add:
case "custom_style":
                           btns = {my_selected_button  : 101,
                                        my_selected_button2: 102
                };
                break;
the name of the buttons that you can select can be found inside the js file.