Heres a couple of links to give an idea of the contrary opinions on it but electrically the answer is yes there may be a Metal Halide bulb that will run on your HPS ballast. You can but try & some people feel safer if the bulb has been packaged as a conversion bulb.

Can you use an HPS bulb in a MH ballast - Marijuana Growing
UK420 > Will Mh Bulb Work With My Hps Ballast & Bulb Holder?
hps ballast w/400w mh bulb???? - Grasscity.com Forums
JackHerer Reviewed by JackHerer on . mh light help Is there a mh bulb I can use with my hps light I've heard of conversion bulbs but I don't know if I need a whole other lighting system to use both mh and hps. Rating: 5