Its definately possible. But I would suggest you use Flouros because the HID is real intense for seedlings/clones. If you do use it keep the light far from the plants to reduce the intensity.
hydroasis tod me the mh conversion buld will be better in the veg stage then switch to standard 400w hps
Well technically hes right. MH are better in veg stage because they put out a more blue spectrum(better for growth), and HPS better for flowering with a red spectrum. What I was saying is that an HID light like HPS or MH are really intense for a young seedling or clone and your better off with flourecent lights until your plants are well established. I usually wait till about 5-6" till I hit em with my HID.