Unless there is something I am missing, I don't think you can since Volt uses the keyword 'for' for it's foreach loop. Only suggestion I would have for you is to implement this within a simple php block. You can execute vaild php blocks inside a Volt block, such as : 
{% for post in posts %}
    <header class="post_header">
        <h3 class="post_title"><a href="{{ url('post/show/') }}{{ post.post_title}}">{{ post.post_title }}</a></h3>
        <span class="post_details"><small>{{ post.post_author }} - {{ post.created_at }}</small></span>
        <h5 class="post_desc">{{ post.post_desc }}</h5>
    </header>
    <?php
    for ($i=0; $i < 10; $i++) {
        echo $myVar;
    }
    ?>
    <article class="post_content">
        <?php echo Parsedown::instance()->parse(substr($post->post_content,0, 500)); ?>
    </article>
{% endfor %}