Add new field to dantocart/opencart checkout page

I require someone to add a new field on the create account page ([login to view URL]) that prompts the user to select how they heard about The Herbjar. The value should be stored in the database, as well as shown in the admin and customer account. The field should be a dropdown, which has a number of options, plus a "Other" option. When "Other" is selected, then another "Other" text field appears underneath. This fields should appear in a new section between "Your Address" and "Password" and be titled "Marketing" The values of the dropdown should also be editable via admin if possible.