etc/adminhtml/system/customer.xml
<?xml version="1.0"?>
<!--
/**
* Copyright © 2017 Magento. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<include xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_include.xsd">
<group id="mundipagg_mundipagg_customer" translate="label comment" showInDefault="1" showInWebsite="1" showInStore="1" sortOrder="20">
<label>Customer Config</label>
<frontend_model>MundiPagg\MundiPagg\Block\Adminhtml\System\Config\Fieldset\Payment</frontend_model>
<include path="MundiPagg_MundiPagg::system/customer/address.xml"/>
</group>
</include>