ContactsApiHelper.php

<?php
require_once('modules/Contacts/ContactsApiHelper.php');

// Since the ContactsApiHelper exists, we'll extend it
// If it didn't we would just extend the SugarBeanApiHelper
class CustomContactsApiHelper extends ContactsApiHelper
{
    // Mimic the SugarBeanApiHelper->formatForApi() class
    public function formatForApi(SugarBean $bean, array $fieldList = array(), array $options = array())
    {
        // First off, call the parent class
        $data = parent::formatForApi($bean, $fieldList, $options);

        // Make sure they requested the top_opp field, or no field restriction at all
        if (empty($fieldList) || in_array('top_opp', $fieldList) ) {
            // Let's just put some filler data in here for now
            $data['top_opp'] = 'Rest in Peas Burger';
        }

        // formatForApi returns the bean formatted as a hash in API format
        return $data;
    }
}