I don't know about ADD/ADHD, but the ability to focus can definitely be improved with training; this is a major benefit of most forms of meditation, for example. Similarly, there's a lot of techniques in Attention Management coming out of the Cognitive-Behavioural Therapy community (and the Meta-Cognitive folks, as well).

Personally speaking, I'd say that if you've got a diagnosis, you ought to talk to the appropriate medical/psychological people for advice; if you're simply interested in improving focus, find a meditation center. YMMV, naturally.