This is most likely a very dumb question and I really don't have an intention on doing this its just curiosity at this point . But what if I put apple cider for flavor into a beer in a fermenter ?
I do it all the time. I make a chocolate apple stout, but I've found that its better to just add frozen concentrate. It's a lot more effective, usually 3 cans for a 5 gallon batch. That gives a slight apple hint, most of it just boosts the ABV.
Yes pasteurization is just heating it to kill off the bugs already in solution. Just means it's sanitary and isn't going to introduce any bad bacteria.