I don't personally have any experience with this, but Bentley over at redwormcomposting.com has. He has a video on creating a worm bin in your garden.

http://www.redwormcomposting.com/videos/vermicomposting-trench-video/

In short, he dug a hole in his garden area, and treated like a large outdoor worm bin.

The best ways to increase worm populations are to one...maintain a mulch on your patch. Two plant a cover crop and add manures in the fall. Till it into your soil in the spring. When your organic content goes up above five percent your native earthworms will appear to amaze you. Keep adding organic content in this manor to reach ten percent organic content. Coffee grounds in any reasonable amount will help the total soil building program. Earthworms love them.

As to your worm farm. The casts are what will add nutrients and trace minerals to your gardens. The red wigglers do not do well away from the raw elements of compost.

Doing the worm farm thing with red wigglers in the garden is a whole lot of un-needed work. Make the soil right and the native worms will appear.

Search "worm trench" or "worm tower" for some interesting reading. I just started a few mini-trenches (wormholes) in small garden plots a few weeks ago. We'll see if the embedded wormeries will thrive and if the castings produced directly near the roots will make a difference. I'll check on the worms every few weeks to see if they need additional food.

Be sure to cover the trench or hole with something so the birds don't eat the worms. Inverted plastic pots work for me.

Andrew