可以添加自定义的Select控件

1.控件dom

<select name="WebSiteTarget" id="WebSiteTarget" class="w1" onchange="editable2(this);">
    <option value="-1">请选择城市</option>
    <option>福州</option>
    <option>厦门</option>
    <option>南平</option>
    <option value="0">(自定义输入)</option>
</select>

2.脚本

<script>
    function editable2(dom) {
        if (dom.value == "0") {
            var newvalue = prompt("请输入", "");
            if (newvalue) {
                addSelected(dom, newvalue, newvalue);
            }
        }
    }

    function addSelected(fld1, value1, text1) {
        if (document.all) {
            var Opt = fld1.document.createElement("OPTION");
            Opt.text = text1;
            Opt.value = value1;
            fld1.options.add(Opt);
            Opt.selected = true;
        } else {
            var Opt = new Option(text1, value1, false, false);
            Opt.selected = true;
            fld1.options[fld1.options.length] = Opt;
        }
    }
</script>

3.效果