Extend Sylius StaticContent model

AppBundle.Document.StaticContent.xml

<!--file: src/AppBundle/Resources/rdf-mappings/AppBundle.Document.StaticContent.xml-->
<type
        xmlns:schema="http://schema.org/"
        typeof="schema:WebPage"
>
    <children>
        <property property="schema:headline" identifier="title" tag-name="h1"/>
        <property property="schema:text" identifier="body" />
        <property property="schema:text" identifier="group" />
    </children>
</type>

config.yml

sylius_content:
    driver: doctrine/phpcr-odm
    resources:
        static_content:
            classes:
                model: AppBundle\Document\StaticContent

routing.yml

# Access to this resource through API
app_api_static_content:
    resource: |
        alias: sylius.static_content
        only: ['show', 'index']
    type: sylius.resource_api

show.html.twig

{#file: /app/Resources/SyliusWebBundle/views/Frontend/StaticContent/show.html.twig  #}
{% extends 'SyliusWebBundle:Frontend:layout.html.twig' %}

{% if cmfMainContent is not defined %}
    {% set cmfMainContent = static_content %}
{% endif %}

{% block content %}
{% createphp cmfMainContent as='rdf' noautotag %}
<div {{ createphp_attributes(rdf) }}>
    <div class="page-header">
        <h1 class="page-title" {{ createphp_attributes(rdf.title) }}>{{ createphp_content(rdf.title) }}</h1>
        {#Add here my new attribute#}
        <h1 class="page-group" {{ createphp_attributes(rdf.group) }}>{{ createphp_content(rdf.group) }}</h1>
    </div>

    <div {{ createphp_attributes(rdf.body) }}>{{ createphp_content(rdf.body) }}</div>
</div>
{% endcreatephp %}
{% endblock %}

StaticContent.php

<?php
// file: /src/AppBundle/Document/StaticContent.php

namespace AppBundle\Document;

use Sylius\Bundle\ContentBundle\Document\StaticContent as BaseStaticContent;

class StaticContent extends BaseStaticContent
{
    /**
     * PHPCR document group.
     *
     * @var string
     */
    protected $group;

    /**
     * @return string
     */
    public function getGroup()
    {
        return $this->group;
    }

    /**
     * @param string $group
     */
    public function setGroup($group)
    {
        $this->group = $group;
    }
}

StaticContent.phpcr.xml

<!-- file: /src/AppBundle/Resources/config/doctrine/StaticContent.phpcr.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mapping
        xmlns="http://doctrine-project.org/schemas/phpcr-odm/phpcr-mapping"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://doctrine-project.org/schemas/phpcr-odm/phpcr-mapping
    https://github.com/doctrine/phpcr-odm/raw/master/doctrine-phpcr-odm-mapping.xsd"
>

    <document name="AppBundle\Document\StaticContent" referenceable="true" translator="attribute" >
        <field name="group" type="string" translated="false" nullable="true" />
    </document>

</doctrine-mapping>