I suppose "huge bankroll" has different meanings to a lot of people. First off, you're not going to be able to do it quickly with no experience. You're going to have to play a LOT of poker to get to the point where you can turn $0 into $10,000.
I started playing poker online when I joined Cardschat last August/September. I haven't dedicated a lot of time to it, other than reading several books and spending time here in the forums - and of course the CardsChat freerolls. Read, Read, Read, Read, Apply what I learned, hope I don't lose my ass off.
That being said, I have managed to build my bankroll up from $0 (never deposited one dime), and have built myself up to just over $100 over two sites (although I'm back down to about $90 after playing badly in several SnGs in a row).
Yes, you can START a bankroll from freerolls, but the money comes from "investing" the freeroll winnings in cash games, SnGs, and the occasional MTT.