Lo resolvi de la siguiente manera, les dejo mi código por si alguien lo necesita en algun momento
$cats = shopp_product_categories();
$cat_ids = array();
$dropdown = array();
foreach ( $cats as $cat ) : $cat_ids[] = $cat->id;
$dropdown[$cat->slug]['group_name'] = $cat->name;
$dropdown[$cat->slug]['group_items'] = shopp_category_products( $cat->id );
endforeach; $products = new WP_Query( array( 'post_type' => 'shopp_product', 'posts_per_page' => -1, 'tax_query' => array( array( 'taxonomy' => 'shopp_category', 'field' => 'id', 'terms' => $cat_ids, 'operator' => 'NOT IN' ) ) ) );
$dropdown['uncategorized']['group_name'] = 'Uncategorized';
while ( $products->have_posts() ) : $products->the_post();
$dropdown['uncategorized']['group_items'][] = array( 'id' => get_the_ID(), 'name' => get_the_title() );
endwhile;