Yes, you can add a new variant to an existing product that is already published on your Squarespace store. In order to do that:
1. Create the new variant manually in Squarespace, e.g. by clicking on "Commerce" >> "Inventory" >> select the product >> "Manage option" >> "Add option" >> select the name of the new variant and click on "Save".
2. In Gelato Dashboard, please go to the Stores section and click "Sync products", after a few minutes you will see a new variant as "Not connected".
3. Connect the new variant to Gelato as shown here.
NoteIf you need to add many variants, it might be faster to create the product from scratch following the steps described here.