I managed to add a phone number and fax number to my contact form by following and modifying the instrucions found in this thread:
http://www.zen-cart.com/forum/showthread.php?t=47525

I need to add several more fields, such as COMPANY NAME, STREET ADDRESS, etc. and after some intense research it appears that in order to auto-populate these fields when a user is logged in the SQL query needs to look in the ADRESS_BOOK table. I think I have isolated the code (see below) that needs to be changed but I don't know enough about how this works to modify the code to correctly include the additional table and fields: entry_company, entry_street_address, etc. Can someone please help point me in the right direction???

<---/includes/modules/pages/contact_us/header_php.php-->

$sql = "SELECT customers_id, customers_firstname, customers_lastname, customers_password, customers_email_address, customers_default_address_id, customers_telephone, customers_fax
FROM " . TABLE_CUSTOMERS . "
WHERE customers_id = :customersID";

Once I get this working I plan on packing up the changed files and submitting this as a CONTRIBUTION for all to use when adding fields to the contact form.