Could you please specify if the product in AliExpress has this variant you want to add in its product page?
If yes and it's the variant that the supplier added after you imported the product, you can add it by using our override feature.
Follow these steps:
Delete the product from Shopify
Add the needed product to the Import list
Click on Action > Select product to override
Search and select product from the pop-up menu and then click "Override"
Confirm the override by clicking on "Push and Override" button.
However, note that manually added variants in Shopify cause product to loss sync and is one of the most common reasons for missing orders. That means the orders of such products do not apper in Oberlo.
Best of luck!