Magento: Merge order to customer account when email exist in checkout

The default behaviour for Magento's default One Page checkout when a customer email already exist in the database is to throw the message "There is already a customer registered using this email address. Please login using this email or enter a different email address to register your account." I would like to change that behaviour. I want the customer to be able to fill in all fields as normal, and if the customer email already exists as an account - i want the order to be merged to his existing account without any hassle. If it exists, there is no need for the "welcome" mail either - only the order confirmation email. For example, the extension "OneStepCheckout" does this. We have this installed, but not using it for different reasons. I have also found this thread where a user is trying to accomplish some of the same goals: [login to view URL], but I only want it to check and merge on the e-mail address.