Sure, you can but it's not a good idea. You have stumbled upon a great find; an 18 year old car with less than 100,000 miles is a heck of a find. This car was never designed for rims and tires of this size. The added weight and turnover ratio's will throw off almost everything in your car from shift points in the transmission to the odometer reading. It puts a tremendous extra strain on the engine, axles, ball joints and suspension. Besides all of this the tires and rims will not fit under the fender wells of the car. To even attempt putting 20's on this car you will first have to lift the car itself which brings on a whole other set of problems such as balance and steering issues.
Sure, the rims might look good but you will sink a lot of money into this car for the project to be successful to the point where the car could be a daily driver.
Besides that consider this; why put $4,000 dollars worth of rims on a 18 year old and probably $2,000 dollar car. Enjoy your great find, keep up with routine maintenance and this car should last for another 18 years.