Can a later model [larger] tank be fitted to a 2009 model Stelvio?
It has been done -- once -- by one of the owner/riders in Europe. I don't remember his name. However, it was not a "drop in" and required a lot of fabrication and rework.
You might want to consider trying to locate one of the auxillary fuel tanks that were available from Italy in that time frame. They mount on the right hand side in that space opposite the muffler. Contact Zoom Zoom (John Henry) for more information on that accessory. He has one on his Stelvio.
Or -- you could buy/mount an accessory fuel tank, similar to those used by Iron Butt riders on their bikes. They come in various sizes and could be mounted on the rear rack. You're probably only looking for two to four gallons more of fuel to get added range.
Or -- as a last resort, you could carry a couple of Rotopaks. They come in various sizes and can be mounted on the sides/rear/top of a rear top case. The down side to them is that you have to stop and pour the gas into the main tank.