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 %}