How to add an existing variation to a product
Turns out I know what you're talking about Nikolaos, and you're in luck! There's a checkbox!

Here's an animated gif that walks you through the steps that I've outlined below:




Navigate to your Content Types
Manage Fields on your Product Display (you may have more than one of these)
Locate the Product Reference or Product Variations Field, click "Edit"
Scroll to the middle and locate the checkbox "Allow users to add existing variations"
...
Profit!