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

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

\Aero\Admin\ResourceLists\OrdersResourceList::extend(function (\Aero\Admin\ResourceLists\OrdersResourceList $list, \Closure $next) {
    // Add it as the first column
    $list->addColumn('Header', function($row) {
        return 'Content';
    });

    // Add it after a specific column
    $list->addColumnAfter('Header', function($row) {
        return $row->reference;
    }, 'customer');

    // Add it before a specific column
    $list->addColumnBefore('Header', function($row) {
        return $row->reference;
    }, 'customer');

    return $next($list);
});

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\ResourceLists\OrdersResourceList::extend(function (\Aero\Admin\ResourceLists\OrdersResourceList $list, \Closure $next) {
    dd($list->getColumns()->map->key());

    return $next($list);
});