Of course they didn't add anything to them! It is an oversimplification to say the least when you write that the "churches have already decided they were the Word of God." Just HOW did these "churches" decide this and based on what? HOW did the "churches" (do "churches" think or do fallible men who make up the Church think and decide?) do this deciding? All they had were individual scrolls with an Apostolic signature that could have been written by an Apostle or someone impersonating an Apostle.
You need to think 2nd century AD. There were no copyright laws in those days. And when there are hundreds of such scrolls around being read liturgically by what magic formula other than the knowledge of the faith that already lived in the Church could they have known what is correct and what is not?
It's like grammar. Do you know all grammatical rules? Most of us don't. But we know what is "correct" English, although we don't always use it. If someone tried to pass a piece of historical find as an American document and used such words as "tyres" and "petrol" or "colour" you'd immediately know that it wasn't an American document.
The church fathers had to have an intrinsic knowledge of what was genuine and inspired and what was false -- and that knowledge did not come from "Bible alone" i.e. the sola scriptura farce.
The gnostic "gospels" sound an awful lot like the real ones, so the task was not as simple as my example with English words or spelling, but a formidable one. This was especially true with the Revelation of John, which took the longest to approve. It took the "churches" over 300 years to "simply" decide to collect all the Word of God. You make it sound like they went to Barnes and Noble and placed their order for "Collected works of God!" Get real.
The compilation of the New Testament is in itself a testament of the Church Tradition, the knowledge of the faith contained in the liturgical life of the Church that pre-existed the New Testament. To put it otherwise: there was never even a possibility of a "sola scriptura" before the end of the 4th century AD.
That knowledge, spiritually-discerned, came directly from the Holy Spirit, not by some natural-minded "intrinsic knowledge." There is one truth, regardless how some try to blur the distinctions. And that one truth we find in Scripture. Just as Paul and the Apostles and the early church were led infallibly by the Holy Spirit, so are all who are born of the Spirit.
And for an excellent understanding of the unity of this truth as carried through from the Old to the New Testaments, there is no finer statements than these:
and...