Question regarding WPML Translations import

Hello,

We started using WPML, and our default site lang is LT-Lithuanian. Now created a second language – EN.

And we want to import product translations to our 20K products.

Is it necessary to have parent import as “New Items” who have “Unique Identifier”, because the WPML addon needs to select Unieuq Identifier to “match” parent import? (image below)

URL: https://puu.sh/IOu75/921cddd1b2.png

The problem is that our "parent" import is created as "Existing Items", so no "unique identifier" can be found in the settings. 

And when testing, the translations aren't importing on already existing products. 

So is it possible somehow to import translations to the existing products if the parent import is not "New Items" ?

Or a different example, we have API connections with other dropshippers, like Bigbuy, they sync products through API, so they don't have import files. 

How to translate? 

I hope I explained clarified this!

and thanks!