Combine Multiple Categories Into One URL Slug

You could override your query with pre_get_posts in functions.php:

function add_all_fruits_to_category($query) {
    $catnames = $query->get('category_name');
    if ($catnames == 'fruits') {
        $query->set('category_name', $catnames . ',bananas,apples,pears');
    }
}
add_action('pre_get_posts', 'add_all_fruits_to_category');