Sorry for the late reply. From what I remember the doors are locked the first time through ng+ but are unlocked on the playthroughs after that. I asked the devs about that because I thought the same thing. They said "It's not currently possible @Zohdin, but it sounds like something we can maybe look into in the future. Once you complete NG+, we wanted you to just have total freedom to play through the levels as you wish."

RTA currently.