Ubercart for drupal, how to add customer ID (identification) field?

I want to ask f开发者_高级运维or the customer to enter his ID when he buys something. Is it hard to add a new field apart from the default: name, phone, address etc ?

From what I understand, Ubercart doesn't use Drupal's Form API, so a simple hook_form_alter won't work. However, here is a blog post with one potential option. The Extra Fields Checkout Pane may also be worth looking into.