I have searched everywhere for a tutorial or guide on how to do this, but i can't find any.
What I'm trying to do is add to my mod. The addition will be a private vault and a door back to Operation: Anchorage.
I don't what to modify the original cell, because it might cause people to not be able to complete it.
I'm sorry if this was already posted, but I'm new here.

You cannot duplicated indevidual exterior cells, instead you must copy the entire worldspace. On the top toolbar click World > Worldspaces then find the one you want, copy it, rename it and use that.

Ok, thanks. I really wanted to do the entire Anchorage thing. But, can I link the first part (The Artillery Guns part) and the second part together? Such as some sort of trigger that makes you go to it, like a COC command?