Write a custom hook for CiviCRM (Drupal)

We are working to setup CiviCRM module for Drupal web site, but have need for some customization. We are using Contribution forms with [login to view URL] as the payment processor for both single and recurring contributions. We have a form setup for donors to setup an ARB subscription, and the form includes some custom fields - one of which specifies a designated project they are giving towards. This custom field value stores fine on the single payments and the first of the ARB payments. Once the subscription is setup and the first payment is made however, Authorize initiates the subsequent payments for the billing cycle. When this is done (via Authorize Silent Post method) no custom field data is available for the project designation field and so the following transactions in the cycle are missing this information. We would like a custom hooks that checks when the Authorize Silent Post comes in to see if it is an ARB transaction, query what the original designation for the Recurring subscription was, and write this value to the custom field for the new transaction. Also, this needs to happen before the confirmation e-mail event is triggered, as this custom field value is attached to the outgoing email to donors. *** If you are not familiar with writing custom hooks for Drupal and CiviCRM, do not bid! ***