How do I add a column to an existing report?

You can use the addColumn() method(s) to add a column to an existing report:

\Aero\Admin\Reports\Implementations\SalesReport::extend(function (\Aero\Admin\Reports\Implementations\SalesReport $report, \Closure $next) {
    // Add it as the first column
    $report->table()->addColumn('Header', function ($row) {
        return 'Content';
    });

    // Add it after a specific column
    $report->table()->addColumnAfter('Header', function ($row) {
        return 'Content';
    }, 'name');

    // Add it before a specific column
    $report->table()->addColumnBefore('Header', function ($row) {
        return 'Content';
    }, 'name');

    return $next($report);
});

If you need to see the keys of the existing columns in order to position your new column appropriately, you can dump them out:

\Aero\Admin\Reports\Implementations\SalesReport::extend(function (\Aero\Admin\Reports\Implementations\SalesReport $report, \Closure $next) {
    dd($report->table()->getColumns()->map->key());

    return $next($report);
});