Overriding Controllers

-- HikaShop version -- : any version
-- Joomla version -- : 3.4
-- PHP version -- : 5.4

Hello,

I have been called by many customers to fix problems that occurred after they updated their hikashop. The problems are that they have customized hikashop versions, with hacks on the controllers or models.

To fight with this, I am looking for any suggestions on how to overrides controllers or models in hikashop.
Is there any good way to achieve this ?

In my current specific case, I only need to override the _doLogin method of the checkout controller.
A subclass of checkoutController would work fine - but how?

Or would that possible with a custom plugin?