Yes, you can add a new variation to an existing listing that is already published on your Etsy store. In order to do that:
1. Create the new product variation manually in Etsy - see the example below.
Open your listing in the Etsy Shop Manager and click on "Edit variations"
Create a new variation, e.g. "Frame" and click on "Add".
Create some options for the newly created variation, e.g. "No frame" and "Black frame".
A new variation will be created.
2. Connect the new variation to Gelato as shown here.