Hooking into the post editing screen for an existing page only

You should really only need to run this on an existing post, since new posts will be created in the manner you specify.

add_action( 'load-post.php', 'custom_content_conversion' );
function custom_content_conversion()
{
    if (! empty($_GET['post']) )
    {
        // Get the post object
        $post = get_post($_GET['post']);

        // If the post object has content, store it in the meta field you are using
        if (! empty($post->post_content) )
        {
            update_post_meta($post->ID, 'your_custom_key', $post->post_content);
        }
    }
}