Not sure about the Custom Field and how that was made or designed that way in your case, normally you want your addresses in the Customer Record, in the Addressbook subrecord because there your address have validation when you add them... doing it with a custom field could be kind of tricky because you will have to make your own validation and at the end you have to write that to the Customer as far as I know in order to be able to select it in the Ship To Field...