On a modified oscommerce shop, there's an option (already present) "ACCOUNT_COMPANY".
When set to "true", the customer can provide company name and VAT number when creating an account and during the checkout.
Problem is, that the data doesn't get displayed in the last step of the checkout or in the order confirmation email.
Also, the ACCOUNT_COMPANY option should be added to a new shop with the order mail changes you'll make.
You'll get access to the site where it's already working and after changing and testing the confirmation mail, you can implement this in the new shop.