怎么在ComboBox的下拉列表中加入DhtmlXTree树形控件

怎么在ComboBox的下拉列表中加入DhtmlXTree树形控件如题:
tree.js
Ext.onReady(function(){
 tree=new dhtmlXTreeObject("treeboxbox_tree","100%","100%",0);   
        tree.setImagePath("codebase/imgs/");   //提明树图标路径
            tree.setEscapingMode("utf8");//设置url编码格式为utf-8
        tree.enableDragAndDrop(0);   //是否允许拖放0/1 允许/不允许
        tree.enableTreeLines(false);//是否显示TreeLines 初始化   
        tree.setImageArrays("plus","","","","plus.gif");//set plus images设置收缩节点图标
        tree.setImageArrays("minus","","","","minus.gif");   //set minus images设置展开节点图标
        tree.enableTreeLines(true),
        //tree.enableAutoTooltips(1);//设置tree的提示,提示内容为item的text
        //tree.setStdImages("book.gif","books_open.gif","books_close.gif");    
       // tree.setXMLAutoLoading("test4.xml");   
        //tree.loadXML("test4.xml"); 
        //tree.enableCheckBoxes(1);//是否显示复选框 0/1 不显示/显示
       // tree.enableThreeStateCheckboxes(3);//允许三种状态的复选框(全选/不选/部分选中)
        //tree.insertNewItem(parentId,itemId,itemText,itemActionHandler,image1,image2,image3,optionStr,childs)   
        tree.insertNewItem(0,1,'区域管理','','','','','','');   
        tree.insertNewChild(1,2,'北京');   
        tree.insertNewChild(1,3,'天津');   
        tree.insertNewChild(1,4,'上海');   
        tree.insertNewChild(1,5,'湖南');   
        tree.insertNewChild(1,6,'广西');   
        tree.insertNewChild(4,7,'徐家汇');   
        tree.insertNewChild(4,8,'百老会');   
        tree.insertNewChild(4,9,'不清楚');   
        tree.insertNewChild(4,10,'哈哈哈'); 
        tree.setOnClickHandler(test);   
        function test(){   
            var text= tree.getSelectedItemText();    
            alert(text);   
        }   


var user_combobox = new Ext.form.ComboBox({
    store:            new Ext.data.SimpleStore({fields:[],data:[[]]}), 
    id:                'person_combobox',
    //hiddenName:        'parent_group',
    readOnly:        true,
    editable:false, //禁止手写及联想功能
    fieldLabel:     '所属部门',
    emptyText:         '请选择部门',
    blankText:        '请选择用户所属部门',
    allowBlank:        false,
    validateOnBlur:    true,
    mode:             'local', 
    triggerAction:    'all', 
    anchor:            '90%',
    tpl:             '<div id="treeboxbox_tree" style="height:200px">123</div>',
    resizable:        true,
    renderTo:"a1"
})
})我用 tpl:             '<div id="treeboxbox_tree" style="height:200px">123</div>',但不成功,请教各位大大