Bluegrass, fescue and ryegrass are three common types of cool season grass. You can typically plant these grasses during mid- to late-fall and the seeds will grow during the winter. Some varieties will die out during the first hot spell of the following year, allowing you to return the lawn to its original grass type.
Overseeding is perhaps the most common way to plant cool season grasses on an existing lawn. You will need to clean your lawn and remove excess thatch prior to spreading the seed, then water the seed in until it begins to grow. Planting cool season grasses on prepared soil is also possible.
When overseeding a lawn, carefully choose a grass type that you can control. Some fescue and ryegrass can grow out of control if left unchecked, choking out your original lawn.