Hi,
To add a field to that form, you first need to edit the file "waitlist" of the view "product" via the menu Display>Views and add your new input field.
Then, you need to add the corresponding field in the table hikashop_waitlist so that the data from the customer can be saved in the database.
Then, you need to edit the file components/com_hikashop/controllers/product.php and modify the insert query in waitlist table to add your new column in the add_waitlist function.
If you want to send an email to the customer, you can modify that same function in order to send that extra email. You can base yourself on the code of the sending of the notification email to the administrator there.