Sure, it's easy to go backstage. But simply put, if you are in plain clothes (i.e. not in costume), security will very likely stop you and ask to see your TM ID before long. It happens all the time backstage, especially when walking between parks.
So, could you? Sure. But know the risks, know the consequences. Universal Orlando is private property and you are guest now. Going back there is not advised at all and you will very likely receive a ban. People have been banned for much less.