AT&T Activation Question (iPhone 5S)

So this question may have been asked and answered a dozen times but I cannot seem to find it.

Basically here is my quandry. If I were to buy an iPhone 5S that is under AT&T but has NOT been activated yet, could I bypass the activation process by obtaining a Nano-SIM from somewhere that is also AT&T branded (possibly buying off Amazon?) and pop it into the 5S? I just want to get past the activation part and play around a bit with the phone before going into a plan and what not.

Thoughts/ideas? And also, how come for some people, he or she is required to input the billing zip code and last four digits of SSN whereas for other people, they do not?