It depends on the model you have. Is yours a tall-tub model with a manual/removable filter at the bottom, or is it a self-cleaning?
You technically could, but it may be more trouble than it's worth. The third level sprayer at the top of the tub, those equipped with it, is basically just a sprinkler. It does help with keeping niblets out of the top of glasses and recesses in the bottoms of bowls and such, but doesn't really add to the cleaning power of the machine as a whole. It won't affect performance or cause strain either; they don't have to change the motor/pump system to accomodate for ones with versus ones without third level sprayers. There isn't enough volume or pressure going to it to cause any decrease at the other points.
The supply tube that comes from the hub of the lower wash arm and goes up the back of the tub would be your key component. The lower end models have the supply tube stop just at the point where the upper rack's wash arm connects. The ones with a third-level sprayer continue up and out to the point where it mounts to the tub ceiling. I added some photos of both just to give reference. You would have to replace the entire supply tube in order to add that sprayer. Because the tubs are all the same, there *should* be a recess and blank screw hole for the tube to mount into the top.
At that point you'd probably spend $60+ buying the supply tube itself, plus the upper sprayer assembly since they don't come paired, and possibly the screw just to secure it to the top of the tub. This process would be the same for the filter model or the self-cleaning model (Point Voyager platform), you'd just have to be sure to use the right part numbers for the right one, as they did update the wash arm system.
