All of them can, but they will only rarely. We need to be very careful here because every offline method of generating something PT-related in return can be hacked one way or the other. So, when you unlock an achievement in Challenge Mode, it will be submitted to our servers which then decide if you get a pack in return and which pack in particular (regular or gold). Several factors are in play here, including a big chunk of randomness, so it is impossible to predict.
The ability to earn pack via Challenge Mode will be unlocked Wednesday.
