Can a new account be added that isn’t currently available to link? And how long does it take to add?

Hello Megan, Welcome to the Community! By new account linked, do you mean bank account? If so as long as it is on the list here then yes you can connect it, usually, it connects within a few minutes to QB and it should give you an option of how far you can go back date wise for transactions or you can start from that day's date.