Hi,
Even if you set a default value to a custom product field, the default value won't be applied to all the elements you already have.
So yes you would have to edit them first to save them.
But in that case, when the value of the custom field is not set in the product, it will actually use the default text of HikaShop, so you could just override the translation of that default text with a translation override:
www.hikashop.com/download/languages.html#modify
There should be only a need to edit the products for which you want a different value.