Moving plugs of your Kentucky Bluegrass into areas that have been damaged or had germination problems is a great way to thicken up the entire lawn without having to start from seed.
Kentucky Bluegrass (and many warm season grasses) will naturally spread out and fill in the holes made by the plugger, and the grass plugs will fill in the surrounding area as well. The ProPlugger is a tool that makes this process incredibly easy. It stores plugs inside itself so you can do about 10 plugs at a time vs the one at a time of a bulb plugger.