the dimmer would have to be between the ballast and light, not the ballast and wall. it would probably end up being more trouble than its worth though becuase you would need the dimmer to be at full power for the light to be able to start, then lower the dimmer to make the light intensity lower. the ballast amplifies the electricity, because the lamp needs more than the 110v coming from the wall, so messing with that might break your lamp. you don't really need to do any of this though, just put the light higher position for the seedlings than your would for a more mature plant. like 4 or 5 feet away.
it'd be kinda cool and convenient if you could wire it up like a normal light with a dimmer. unfortunately HID lights are far from normal lights.