Yes, you can add roughly the same number as the tank from which the fish came. I would err on the lower side of that number.
Second, you can cut the media up. There's no sense in keeping it all in one big piece. Dirty side should face the new media as much as possible.
You won't be able to keep it forever, but it will last a much longer time than the manufacturer will claim. They want you to buy more of their product, so they will say things like "replace every 2-4 weeks". That's just ludicrous! Replace when it starts to completely disintegrate, and not much sooner than that.