Edit in JSFiddle

Ext.ns('Ext.ux.form.HtmlEditor');

Ext.ux.form.HtmlEditor.HR = Ext.extend(Ext.util.Observable, {
    init: function(cmp){
        this.cmp = cmp;
        this.cmp.on('render', this.onRender, this);
    },
    onRender: function(){
        this.cmp.getToolbar().add([{
            iconCls: 'x-edit-bold', //your iconCls here
            handler: function(){
                this.cmp.insertAtCursor('<hr>');
            },
            scope: this,
            tooltip: 'horizontal ruler',
            overflowText: 'horizontal ruler'
        }]);
    }
});
Ext.preg('ux-htmleditor-hr', Ext.ux.form.HtmlEditor.HR);

Ext.QuickTips.init();
new Ext.Viewport({
    layout: 'fit',
    items: [{
        xtype: 'htmleditor',
        plugins: ['ux-htmleditor-hr']
    }]
});