How do I add a lens to an existing report?

You can use the 
addLens() method to add a lens to an existing report:
\Aero\Admin\Reports\Implementations\SalesReport::extend(function (\Aero\Admin\Reports\Implementations\SalesReport $report, \Closure $next) {
    $report->addLens(\Aerocargo\Acme\Lenses\TotalRevenueLens::class);
    return $next($report);
});
class TotalRevenueLens extends \Aero\Admin\Reports\ReportLens
{
    protected function value(): \Aero\Cart\CartPrice
    {
        $totalRevenue = \DB::query()
            ->fromSub($this->query, 'results')
            ->selectRaw('SUM(results.revenue + results.tax) as total')
            ->value('total');
        $currency = request()->store()->currency;
        return new \Aero\Cart\CartPrice($currency, $totalRevenue * 100);
    }
}
Where the lens added extends 
\Aero\Admin\Reports\ReportLens:
class TotalRevenueLens extends \Aero\Admin\Reports\ReportLens
{
    protected function value(): \Aero\Cart\CartPrice
    {
        $totalRevenue = \DB::query()
            ->fromSub($this->query, 'results')
            ->selectRaw('SUM(results.revenue + results.tax) as total')
            ->value('total');
        $currency = request()->store()->currency;
        return new \Aero\Cart\CartPrice($currency, $totalRevenue * 100);
    }
}
$this->query is a copy of the report query, the 
value method must return a string (or stringable).