Well, there's no such thing as a magazine tube extension for a Mossberg 500, so let me answer this a different way.
The 5-shot Moss 500, the 8-shot Moss 500, the short 590 and the long 590 ALL have interchangeable barrels and magazine tubes, but you MUST change [b]both the barrel and magazine tube[/b] for it to work.
Example: My 8-shot Mossberg 500 began life as a 5-shot 500. A buddy of mine wanted a 5-shot also, so I paid the difference in price and he bought the 8-shot one. I then swapped the barrels, magazine tubes, and magazine springs. I also swapped mag followers, but that was unnecessary. Then I had the 8-shot and he had the 5-shot, but we both kept our respective receivers and serial numbers.
In the same way, the 590 parts can go on a 500, and vice versa. You just have to change the whole front end, since the 500 and 590 barrels attach differently.