Editing specific fields from a POSTed form in a plugin
You will want to use PHP's foreach to loop over each of the items (using $i here to keep track of the index). Then if the current item has a start date, do whatever it is you need to edit it inside the loop. Once you've finished iterating over each item, you will need to set that back to $_POST['items'], so that the MultiAdd plugin can pick up your modified array as usual, using craft()-&gt;request-&gt;getPost('items').

public function actionReorder() {
    // ... a bunch of stuff relating to the cart ...

    // loop through all the inputs for line items and amend any startDate
    $items = craft()-&gt;request-&gt;getPost('items');

    foreach ($items as $i =&gt; $item)
    {
        if (isset($item['options']['startDate']))
        {
            $items[$i]['options']['startDate'] = '???';
        }
    }

    // re-set back to $_POST
    $_POST['items'] = $items;

    // ok now pass it all over to the multiAdd controller...
    craft()-&gt;runController('multiAdd/multiAdd');
}