Of course it's possible. Plenty of room in the trunk for some home charged batteries. Attach a motor to each rear wheel. Control by tapping off the signal from the accelerator pedal. Go for it. Bring a large arc welder.
Getting the Prius to recharge the additional batteries would be a much more cerebral task. But there are a couple groups doing it. Try following their lead. At least one of them has posted here a couple times.
And of course, any attempts to do this will void your warranty in a big way. Be careful, don't break your car. Check into your state's laws on what modifications are street legal before starting. Save lots of money before starting, you'll need it.
Define your goals carefully ahead of time and plan your project accordingly. You can probably improve acceleration and performance in slippery conditions. Unless you're very careful, you'll reduce gas mileage quite a bit. I don't know if it's even possible to improve gas mileage this way. You'll lose your trunk space (do you ever need it?). There's a good chance you'll make the car rust quickly unless you pay careful attention to sealing everything.
Good luck. If you try this, take lots of pictures and let us know how it goes.