Batch Adding of Keywords

To save time, it would help to be able to add new
keywords to a large group of existing pics that may not
be within the same album.
Say I have photos of houses, categorized in albums by
area. Later I realize that I want to add "2 car garage"
to all the appropriate pics. I'd like to be able to pick the
appropriate pics, and then add the keywords in one
operation, rather than manually adding the keywords to
each individual pic.