How do I add a filter to an existing resource list?

You can use the addFilter() method to add a filter to an existing resource list:

\Aero\Admin\ResourceLists\OrdersResourceList::extend(function (\Aero\Admin\ResourceLists\OrdersResourceList $list, \Closure $next) {
    $list->addFilter(\Aero\Admin\Filters\Order\OrderStatusAdminFilter::class);

    return $next($list);
});