Add field to customer billing address, for example VAT ID

Hi, what is the best way to add a field to customer (e.g. VAT or company ID) so that it would show in checkout and also in customer profile, invoice etc.

Currently it seems that there are some hardcoded fields in customer address and I need to add other fields, since they are required on invoices in my country.

Thanks,
Robert

I just received reply from Telerik, it is described in documentation, I must have overlooked it:

www.sitefinity.com/.../persisting-the-values-of-the-fields-in-the-cartorder-instance