So, I have completed the mission that brings you to this building and gains you access to the elevators. Immediately after I went and gathered the free fall leg armor in the building. While I was on my way back down I discovered a floor with several doors on it, all of which said that the door requires a key.

So I was wondering, is there a key I can acquire for these doors, or are these doors just a biproduct of the mission and can't be opened?

Byproduct of the mission. You will see what is inside soon enough. (Cue jaws music)

You'll need to speak to McReady to get to those doors. Looked it up. Was sorely disappointed by spending a good hour hunting for the ID card. Frustrated, went online to find the ID card location.

No chance for me to find out what's down there. I travel alone.

Mass fusion locked door is related to the main quest. You will come here later eventually when the quest makes you come here.

I've already completed the main story quest that brings me there, I'm literally one mission away from beating the game. The doors I am referring to are really high up, they're the levels the enemies spawn on during the elevator ride down. Those doors are so high up I can't see any way to get mccready up there.

EDIT: I just looked it up too, are you guys sure you aren't thinking of the mass pike interchange mission? I don't see any reference for McCready in the mass fusion building.

I've tcl'd through those high up doors and there is nothing there. So maybe future content.

In the video at the 1:00 mark, the author goes through the mass fusion building to get an item. I guess this would be a spoiler so I'll be vague.

https://www.youtube.com/watch?v=dPQHOlWRiZY

I will note, while I was on the elevator ride down there was a legendary enemy that spawned there. I didn't kill him because I figured I'd just go back up there and kill him for his legendary item drop later. As the elevator went down he ran through one of the doors, and when I got to the bottom floor he was there. SO perhaps it's just there so the enemies that you don't kill can follow you? Who knows, maybe time will tell what they are for.