I don't think it's necessary to add worms. There are probably already some worms, even if not very many. As you add more organic matter, they'll have more food and the population will increase. If there isn't enough food for them, adding more worms won't help, anyway.

When I bought my house, I hardly ever saw a worm anywhere. Now, they're everywhere.

Hi Volleyman,

Save your money on the worms. They will come on their own when the conditions in your lawn are proper for them to survive.