Yes, you can add a new variant to an existing product which is already published on your Shopify store. In order to do that:
1. Create the new variant manually in Shopify, e.g. by clicking on "Add variant" or by duplicating an existing variant - see screenshots below.
2. Connect the new variant to Gelato as shown here.
Note: If you need to add many variants, it might be faster to create the product from scratch following the steps described here.