You have access to make those changes in v7 as well, just have to mark that class to show as a "system table" in the cms_class table. Then when you go to Site Manager>Development>System Tables, you'll see that table available and you can then add the field to the table from there with the proper control. It may also require some custom work on the checkout process since it is v7 but I can't say what that is as I only have 1 client on v7 now and they don't have ecommerce all the rest have upgraded to newer versions.