How do I add a filter to an existing report?

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

\Aero\Admin\Reports\Implementations\SalesReport::extend(function (\Aero\Admin\Reports\Implementations\SalesReport $report, \Closure $next) {
    $report->addFilter(\Aerocargo\Acme\Filters\MinMaxStockFilter::class);

    return $next($report);
});