// Convert a JSON field to an array
$registry = new JRegistry;
$registry->loadString($item->jsonfield);
$item->jsonfield = $registry->toArray();

// Convert an array field to a JSON
if (isset($data['jsonfield']) && is_array($data['jsonfield']))
{
    $registry = new JRegistry;
    $registry->loadArray($data['jsonfield']);
    $data['jsonfield'] = (string) $registry;
}