Add Customers Date of Birth to all Order Confirmation eMails

We have a Zen Cart site where the customer enters their date of birth on registration. I need this date of bith to be included in all new order confirmation emails. At present, the the order confirmation emails only includes their name, address, email, telephone and IP. I need it to include their date of birth (which is already a database field). When you're finished, I need a detailed list of all the file hacks and changes you've made to our site files.