These are v3 steps
1. use phpadmin or other php script to add the fields you desire to CubeCart_customer table.
2. update code admin/customer/index.php to be able to view/edit those fields
3. update code in includes/content/reg.inc.php to enable those fields for the customer
4. update reg.tpl in all skins folders include them on the reg page
5. update profile.tpl in all skins folders include them on the reg page
6. update code in includes/content/profile.inc.php to write customer's updates
that's it, I think. If i missed something I am sure someone will add their two cents