I'm really surprised that Dutch did not mention CMH (ceramic metal halide) bulbs. These put out alot of UV in comparison to even MH bulbs.

One of the cheapest and easiest ways to add UV is the addition of some reptile bulbs, either CFL or linear flourescents, to your HPS. But as pointed out by Dogsnova on another thread, you may get the same benifits from the use of ceramic incadescents to increase far red, without the health risks associated with UV.