Hi,
1.
HikaShop allows for different layouts on the product page. The tabs are part of the tabular layout. If you create custom product fields, they will display on the right side of the page in the default layout. And if you use the "tabular" layout, they will display in an extra tabs of the tabular layout.
So the way HikaShop is built, to have an interface to be able to have custom fields as extra tabs, it would require to have that capability added to the layout mechanism of the tabular layout, since it wouldn't make sense for other layouts to have tab options where there are no tabs mechanism in the layout.
In most cases where extra tabs are needed for custom fields, this is a matter of a few minutes / hours of customization of the show_tabular layout for a PHP developer (even a beginner should be able to do it) and then the merchant using the shop doesn't have to mess with this.
So to the question
why doesn't hikashop have product tabs as a default which can be enabled/disabled?
, I suppose I would say that it's because the tabs system we have is one where the tabs are added / removed by HikaShop automatically based on the features activated in the backend (for example if you activate the vote & comment system, it wil add 2 tabs to the tabular layout) and we didn't see how to easily linked custom fields with extra tabs.
Note however that I'm not familiar with the tabs system in MijoShop. And I couldn't find any information about it online. If you could tell us more about how this works, maybe we'll have an idea on how to improve things.
2. The custom fields won't be filled by the import system. Supposing that the tabs content is stored in different columns in the mijoshop_product table, you could run a MySQL query via your PHPMyAdmin after the import to populate the custom fields in HikaShop:
UPDATE #__hikashop_product as hkp INNER JOIN #__hikashop_mijo_prod as hkm ON hkp.product_id=hkm.hk_id INNER JOIN #__mijoshop_product as mjp ON hkm.mijo_id = mjp.product_id SET hkp.xxx = mjp.yyy;
where #__ is the table prefix of your website, xxx the column name of the custom field in HikaShop and yyy the column of the mijoshop_product table where the data you want to import is stored.