Import Category For Multiple Stores

We have CS Cart Ultimate with 3 storefronts.
Now we want to update the product categories by using a CSV import.
So we have 3 stores:
Shop 1 - COM
Shop 2 - DE
Shop 3 - NL
We share products across these shops, so a product is listed on all three shops. In their own language, and own category.
We have "Shop 1 - COM" as our main store, thus the product's owner.
When we import, we can only use the "Shop 1 - COM" store to import categories. Setting either the language to nl or de, or changing the shop doesn't work. Also When we change the shop, nothing happens. That is because that shop is not the product's owner.
But when we keep the main store in the CSV, the new categories are added to that shop.
Now what we need, is the possibility to add categories to a product, even if it is not in the product's owning store.