It can be done but not easily or cheaply. Since the barrel extension on the 500 screws directly into the end of the magazine, you can't add an extension like you can on the 590 where the barrel extension slips over the magazine. The magazine on the 500 is, however, screwed into the receiver and can be removed. Mossberg makes a model called the Persuader that has an eight round magazine. You could replace the shorter mag with the Persuader mag but you would also have to buy a Persuader barrel to fit the longer magazine. In the long run, unless you could find the necessary parts used and cheap, it wouldn't be cost effective.
"Those who hammer their guns into plows will plow for those who do not."