they make conversion bulbs so that you can use the HPS spectrum bulb in a MH ballast.
then they have conversion bulbs to use the blue like MH in an HPS ballast.
the bulbs are not cheap. here is a link. im sure they are cheaper somewhere else but they do make them.
http://www.4hydro.com/lighting/HID-conversion-bulbs.asp
the best option is to have an HPS ballast because they are more efficient than a metal halide ballast. im dont know your setup but you can get cheap HPS ballasts on ebay. you just have to search to find one. a used warehouse light converted to grow light is an inexpensive way out, but electronic ballasts are better. depends on what you want to spend.
check out the link in my sig about warehouse lights.