I believe you can add items to your order as long as it is marked as "open" (not in shipping process), but I've never done that myself
So far Hitomi's been releasing an album every year, but I think after she changed her style so much, her sales dropped a lot, so maybe avex will try to get more money by releasing another single... so I'd put my bet on 2007 for a new full album. I wouldn't be surprised if they'd release crossover II album around X'mas though. Ok, well, I'm definitely the best strategist in J-pop industry /sarcasm