Hey. The kit's are pretty versatile, actually. You can retrofit them into an existing hood if you wanted. However, I'm not entirely certain that you'll be able to use the AH reflector inside the hood. The reflector is one of the reason's that the kit works so well. I think, compared to a regular, rectangular reflector, it reflects like 125% of the light. You'd have to check the site for that info.
Or, you could build yourself a custom hood. It's dirt cheap to do and insanely easy. I think, after paying for paint, lumber and finishing nails, I spent like $15 (probably less, actually) on the actual components. It takes very little time to assemble and actually looks really good. My tank stand and trim are black so I just painted the hood black and everything matches.
I'm sure you could assemble the unit and hang it on it's own, if you wanted. I guess you'd have to use chains or wire and hang it from the ceiling. You'll want to cap off the ends of the reflectors because, man, those puppies reflect a lot of light.
Check the site. It gives you all the info you need on how to upgrade. The guy that runs it is also extremely helpful and will answer any and all questions you have.
Good luck!