Yes it's possible to build a
bankroll from freeroll winnings, it takes time and it really depends on how big a bankroll you want.
I generally play for fun I'm never going to be good enough to earn serious money playing poker, but in the last 4 years I have taken some fairly modest freeroll winnings to an online bankroll of around $5000.
The way I did it was to find a site that would allow you to withdraw winnings even though you haven't deposited, I think at the time I used a merge skin. After I'd won a bit I moved that out to
neteller, from there I could move money around sites that would only allow withdrawals after you have deposited to free up my freeroll winnings there. Another good source of funds has been forum freerolls and money added buyins. I've moved away from freerolls now as they are so frustrating dealing with the all in maniacs. Having said that I won $45 on a new depositors freeroll on Littlewoods poker last night, but thats rare now.
As for depositing well I'm glad I never started that way I would have lost money to start with and I hate losing money it would have probably been put off playing and I get a huge amount of enjoyment out of this frustrating silly little game