Yes, you can add a new variation to an already published product on your Wix store.
Add the new Variation in Wix
First, you will have to add the new variation in Wix. This can easily be done by adding more Product Options when editing your product in Wix.
You can read more on how to do so in the Wix Help Center article Wix Stores: Adding and Customizing Product Options.
You can also set different prices for your variants by clicking on edit in Manage Variant.
Don't forget to save your changes!
Sync products in Gelato
Once the new variation is created in Wix, navigate to the Stores section and select your Wix store. Click on the "Sync products" button is located on the top right of the page.
After a while, you will see a new variant as "Not connected".
Connect the new variation in Gelato
You will now be able to connect the new variation to a product in Gelato as shown here.