Yes, they can do that. But your comment above that "[a]dding the second flight shouldn't change the points required, though there could be a difference in taxes" is not necessarily true. The price could change between now and when you add this other segment, and you may have to pay the difference.

If the second flight is within the same zone for pricing purposes and it's all on partner airlines, you should be good unless AC changes the award chart. Historically, they haven't done that often, but it's a new world, and who knows what they'll do.