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.