WordPress is a php based CMS. Orchard is ASP.NET.
Short answer: you will not find any automatic converter.
However, I am going to bet that you are mainly interested in the appearance of the WP theme that you are talking about, and you really do not ask about the transfer functionality. Look at index.php and single.php in your WP topic to find basic HTML markup. Take a look at styles.css or any other basic CSS files to find basic styles. Then use Frooth . This makes it easy to change the layout and styles using the graphical interface. If you know the basics of HTML and CSS, this should be more or less a job of cutting and pasting.
Good luck