I had thought a lot about this a few years ago. It would not be hard to make the extenstions, and even have them move up and down with the factory ones, the problem is the tank fill auger. It pivots on a U joint, and that U joint is basically at max operating angle when the auger is all the way up and tank is full. To push it farther as is isn't a good ideal. You would need to rework that auger to make it work. Also note that the first place you will fill up is the back right corner where the clean grain auger comes up. Getting that figured out as well as making it accessible will be an issue as well.
That's my $.02, take it for what its worth. Get a better grain truck/cart driver.