Craft 3 js - extend the field layout designer field settings
If someone has a better approach to solve that please tell me because my method right now fells really wrong and like a hack that might be troublesome in the future. I would like to accept the answer of someone else - until then I'll use the following

onKeyDown: function(ev){
    ev.preventDefault();
    var btn = $(ev.target);
    if(btn.data('menubtn')){
        var menu = btn.data('menubtn');
        var options = menu.menu.$menuList;
        if(options.find('.myClass').length === 0){
            options.append('&lt;li&gt;&lt;a class="myClass" data-action="myCustomDataAction"&gt;Another Button wuhu ^.^&lt;/a&gt;&lt;/li&gt;')
        }
    }
},
init: function () {
    var e = this;
    var icons = $(".settings.icon");
    $.each(icons, function(index, item){
        var menuBtn = $(item).data('menubtn');
        if(menuBtn){
            var $menu = menuBtn.menu;
            $menu.on('show', $.proxy(e, 'onShowMenu'));
            e.addListener($(item), 'mousedown', 'onKeyDown');
        }
    });
    // add the event listener
    Garnish.$doc.on('click', '.myClass[data-action="myCustomDataAction"]', this.showHoverMenu.bind(this));
},