Filter the analyzed content

function sp_content_analysis_content($content, $id) {
        //$content = default WP editor
        //$id = current post ID
        //Example to add your custom field to content analysis
	$cf = get_post_meta($id, 'my-custom-field', true);
        $content = $content.$cf;
        return $content;
}
add_filter('seopress_content_analysis_content', 'sp_content_analysis_content');