I bought some menthol crystals today, and tried placing a small shard on top of the cartridge material. It melted in about 2 puffs, and added a nice menthol flavour. And when I say small shard, I mean SMALL. I can't give you an idea of size, but teeny would be a good description. It lasted for the entire cartridge life too. Be careful, too many pieces will turn you into Frosty the snowman.
If you're in Canada, Shoppers drug mart will sell you menthol by the gram. 5 grams = 84 cents!