如何为element-ui的Select和Cascader添加弹层底部操作按钮

<ul class="el-cascader-menu__list" style="border-top: solid 1px #E4E7ED;padding:0">
  <li class="el-cascader-node" style="height:38px;line-height: 38px">
    <i class="el-icon-plus"></i>
    <span class="el-cascader-node__label">新增商品分类</span>
    <i class="el-icon-arrow-right el-cascader-node__postfix"/>
  </li>
</ul>

/**
 * 为element-ui的Select和Cascader添加弹层底部操作按钮
 * @param visible
 * @param refName  设定的ref名称
 * @param onClick  底部操作按钮点击监听
 */
visibleChange(visible, refName, onClick) {
  if (visible) {
    const ref = this.$refs[refName];
    let popper = ref.$refs.popper;
    if (popper.$el) popper = popper.$el;
    if (!Array.from(popper.children).some(v => v.className === 'el-cascader-menu__list')) {
      const el = document.createElement('ul');
      el.className = 'el-cascader-menu__list';
      el.style = 'border-top: solid 1px #E4E7ED; padding:0; color: #606266;';
      el.innerHTML = `<li class="el-cascader-node" style="height:38px;line-height: 38px">
<i class="el-icon-menu"></i>
<span class="el-cascader-node__label">商品分类管理</span>
<i class="el-icon-arrow-right el-cascader-node__postfix"/>
</li>`;
      popper.appendChild(el);
      el.onclick = () => {
        // 底部按钮的点击事件 点击后想触发的逻辑也可以直接写在这
        onClick && onClick();

        // 以下代码实现点击后弹层隐藏 不需要可以删掉
        if (ref.toggleDropDownVisible) {
          ref.toggleDropDownVisible(false);
        } else {
          ref.visible = false;
        }
      };
    }
  }
},

<el-cascader
    :options="cascaderOptions"
    v-model="cascaderValue"
    @visible-change="v => visibleChange(v, 'cascader', cascaderClick)"
    ref="cascader"
/>