I'm pretty sure you can't but not 100%.
Put it this way - I had some clones that had been under 12/12 for too long, most eventually died, even thought they rooted. I have just about managed to bring 5 around - one is doing well now and can be seen in my journal. I have another four I am playing with that might just about manage to live but I won't be including them in my grow - think they'll be going outside on a random guerrilla grow somewhere - I'll go back in a few months and see if they did anything.
there is a strain called flo which is a reproducer. Once it has flowered you remove the buds and it will re-flower in 6 weeks (or whatever flower period it took). You cannot put it back into veg though I don't think.