Can you yes you can do anything you want. Most ballasts these days are multi watt ballasts for example 250w-400w-600w or 400w-600w-1000w. This i s so you can use different watt bulbs for different stages of plant life and growth, or say if you plant to upgrade to a larger setup later all you need to do is buy a larger watt bulb and not a whole new light setup. It is not ment for running a higher watt bulb on a lower wattage, this is not recommended as stated by many others already. Most ballasts also come with a "Super" setting that slightly overdrives the bulb normally by around 20-100 watts depending on bulb size. This will also shorten bulb life but is nit as bad as underdriving a bulb. So concider overdriving an even trade off for a little more watts.
