Hi,
You could add a new field and have the import import the id in that new field. Then, you could use a mass action to update the product_id with the value in the custom field so that the system would be able to use the same ids.
Regarding the 404 errors, that's because you've activated the setting "remove product and category ids from URLs" in the HikaShop configuration and that your products have special characters in their name and that you don't have aliases without special characters for them and that you have the unicode slug setting of Joomla deactivated.
So when creating the URL of the product, the system can't use the aliases since there are none, thus it uses the product_names instead but the special characters are replaced as non-UTF8 characters by Joomla's SEF.
Then, when HikaShop has to read the URL to guess the product, it doesn't have the id, and the alias in the URL doesn't match with either the product name nor the product alias and thus you get an error.