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);
});