Go to your themse’s local.xml and place the code
Overriding A System.xml file for adding addition settings to an exist module in a new module. Module name is (Namespace_Modulename) Note : Change as per requirement like change groupname,sectionname,fieldname.
Then after for saving these datas into database.Go to register.phtml (Path can be :- app/design/frontend/default/bootstrap/template/opc/customer/form/register.phtml
Open list.phtml and any other related on which you want to display the addtocart button and quantity box.