To be quote honest, you should not be adding *any* worms to your garden, especially not composting worms.
Composting worms are of the epigeic variety, Greek for "on the soil." These are not soil dwelling worms and will not help to improve or aerate your soil and should not be added to anything but the lightest, fluffiest soil possible.
If you garden soil is lacking in worms, then you should add organic matter like vermicompost, compost, or manure to your soil which will help attract soil-dwelling worms to your garden.