How to show user attributes in the customer's table in Shopware 5?

Yes, it is possible to extend the listing view. In my example billingPhone and shippingPhone have been added.

First you have to subscribe to the following events:

'Enlight_Controller_Action_PostDispatchSecure_Backend_Customer' => 'onCustomerPostDispatch',
'Shopware_Controllers_Backend_CustomerQuickView::getModelFields::after' => 'addPhoneOnFields',
'Shopware_Controllers_Backend_CustomerQuickView::getListQuery::after' => 'addPhoneOnSelect',

Then have to load the data:

public function addPhoneOnSelect(\Enlight_Hook_HookArgs $args)
    {
        /** @var \Shopware_Controllers_Backend_CustomerQuickView $subject */
        $return = $args->getReturn();
        $return->addSelect('billing.phone as billingPhone');
        $return->addSelect('shipping.phone as shippingPhone');
        $return->leftJoin('customer.defaultShippingAddress', 'shipping');
        $args->setReturn($return);
    }

    public function addPhoneOnFields(\Enlight_Hook_HookArgs $args)
    {
        $return = $args->getReturn();
        $return['billingPhone'] = ['alias' => 'billing.phone', 'type' => 'string'];
        $return['shippingPhone'] = ['alias' => 'shipping.phone', 'type' => 'string'];
        $args->setReturn($return);
    }

Then you have to extend the view with the data

public function onCustomerPostDispatch(Enlight_Controller_ActionEventArgs $args)
    {
        /** @var \Enlight_Controller_Action $controller */
        $controller = $args->getSubject();
        $view = $controller->View();
        $request = $controller->Request();

        $view->addTemplateDir(($this->container->getParameter('reply_extend_customer_overview.plugin_dir') . '/Resources/views/'));

        if ($request->getActionName() == 'load') {
            $view->extendsTemplate('backend/swag_extend_customer_overview/view/detail/window.js');
            $view->extendsTemplate('backend/swag_extend_customer_overview/model/quick_view.js');
            $view->extendsTemplate('backend/swag_extend_customer_overview/view/main/customer_list.js');
        }
    }

You have to extend the model for the listing like this

// {block name="backend/customer/model/quick_view/fields" append}
{ name: 'billingPhone', type: 'string', useNull: true },
{ name: 'shippingPhone', type: 'string', useNull: true },
// {/block}

And the listing

// {namespace name=backend/customer/view/main}
// {block name="backend/customer/view/main/customer_list" append}
Ext.define('Shopware.apps.SwagExtendCustomerOverview.view.main.CustomerList', {
    override: 'Shopware.apps.Customer.view.main.CustomerList',
    alias: 'widget.customer-list',

    configure: function() {
        return {
            displayProgressOnSingleDelete: false,

            /* {if {acl_is_allowed privilege=delete}} */
            deleteButton: true,
            deleteColumn: true,
            /* {else} */
            deleteButton: false,
            deleteColumn: false,
            /* {/if} */

            /* {if {acl_is_allowed privilege=detail}} */
            editColumn: true,
            /* {else} */
            editColumn: false,
            /* {/if} */

            /* {if {acl_is_allowed privilege=update}} */
            addButton: true,
            /* {else} */
            addButton: false,
            /* {/if} */

            columns: {
                active: { header: '{s name="active"}{/s}', width: 50 },
                id: { header: '{s name="id"}{/s}' },
                customerGroup: { header: '{s name="column/customer_group"}{/s}' },
                shop: { header: '{s name="shop"}{/s}' },
                number: { header: '{s name="column/number"}{/s}' },
                email: { header: '{s name="email"}{/s}', renderer: this.mailRenderer, flex: 2 },
                salutation: { header: '{s name="salutation"}{/s}', renderer: this.salutationRenderer },
                title: { header: '{s name="title"}{/s}', width: 70 },
                company: { header: '{s name="company"}{/s}' },
                firstname: { header: '{s name="column/first_name"}{/s}' },
                lastname: { header: '{s name="column/last_name"}{/s}' },
                zipcode: { header: '{s name="zip_code"}{/s}' },
                city: { header: '{s name="city"}{/s}' },
                firstLogin: { header: '{s name="first_login"}{/s}' },
                lastLogin: { header: '{s name="lastLogin"}{/s}' },
                accountMode: { header: '{s name="column/accountMode"}{/s}', renderer: this.accountModeRenderer },
                lockedUntil: { header: '{s name="lockedUntil"}{/s}' },
                birthday: { header: '{s name="birthday"}{/s}' },
                billingPhone: { header: '{s name="default_billing_phone_label" namespace="backend/plugins/xxx/translation"}Billing phone{/s}' },
                shippingPhone: { header: '{s name="default_shipping_phone_label" namespace="backend/plugins/xxx/translation"}Billing phone{/s}' },
            }
        };
    },
});
// {/block}