Hi,
I'm not sure what your goal is.
Custom fields of the table "order" can only be selected/inputed by the user during the checkout process. Once the order is created, he can see the custom fields filled if you allow for that, but he cannot modify them. Only you can do it from the backend.
So if your question is that you don't see the custom field during the checkout process, then please check your Checkout workflow, under the Checkout tab of the HikaShop configuration and make sure you have the "Fields" view added with that custom field selected.
If your question is that you want the customer to select/input something on his order details page, after the checkout is completed, then there is no easy solution. It would require some development to allow for that.