God's name originally was in the Greek Scriptures because it would be inconceivable that the Bible writers did not use God's name, especially when quoting from parts of the "Old Testament" that contained the name. If God's name did not belong in the New Testament, then why did Jesus tell us to pray for its sanctification? (Mt. 6:9)
The manuscripts we have today were copies of copies, written hundreds of years after the originals, and may well have been changed when the name became a hated “Jewish” name to “Christians” around 135 A.D.
Scholars know for a fact that words and whole phrases have been added or removed from various ancient copies of the New Testament, but that doesn't make the New Testament unreliable, since many or most of the additions or subtractions can be recognized or restored.
It should not be considered terribly wrong to restore, for the sake of clarity if nothing else, the name we know belongs there. Many other respected Bibles add personal names to their translations for clarity when they weren’t actually in the New Testament text to begin with. There are many examples of this, but to examine just one:
John 12:41 says in the available manuscripts: “Isaiah said these things because he saw his glory and he spoke about him.”
Some respected Bibles replace “he” or “him” with “Jesus”. See, for example, NIV; NJB; and NAB (’70).
Some condemn the New World Translation Bible for restoring God's Name some 237 times in the Christian Greek Scriptures where it was likely there originally. Places where the Christian Greek Scripture writers quote the earlier Hebrew Scriptures containing God's name. Many translators have done this. Really, shouldn't the condemnation be directed toward the English Bibles' removal of God's Name nearly 7,000 times in the Hebrew Scripture (when the Name is clearly there)?
For much more, see:
"Jehovah" in The New Testament (SFBT)
NWT - Criticism by Zondervan's So Many Versions? - "Jehovah" in the New Testament (DNWT)
God's Name (SFBT)
Defending The New World Translation - Archive (DNWT)