I had a requirement like customer support will able to login as any customer. Here is the code snippet i used to force customer support to login as normal user.

  $customerId = (int) $this->getRequest()->getParam('id');
  $customer   = Mage::getModel('customer/customer')
                    ->load($customerId);

  $userSession = Mage::getSingleton('customer/session');
  $userSession->setCustomer($customer);
  Mage::dispatchEvent('customer_login', array('customer'=>$customer));

  $this->getResponse()->setRedirect(Mage::getUrl('customer/account'));
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