Summary of Jehovah's Witness Teaching: Jehovah God is the Creator (Revelation 4:11). He is without beginning or end, and he is almighty (Psalm 90:2). Jesus, on the other hand, had a beginning (Colossians 1:15- 16). Colossians 1:15 calls Jesus “the image of the invisible God.”
Extracts from the 15 September 2005 Watchtower article (pp. 4-7) ‘Who Is Jesus Christ?’
Since all created things had a beginning, there was a time when God was alone. Countless ages ago, however, God became a Creator. Who was his first creation? The last book of the Bible identifies Jesus as “the beginning of the creation by God.” (Revelation 3:14) Jesus is “the firstborn of all creation.” That is so “because by means of him all other things were created in the heavens and upon the earth, the things visible and the things invisible.” (Colossians 1:15, 16) Yes, Jesus was the only one directly created by God himself. Therefore, he is called God’s “only-begotten Son.” (John 3:16) The firstborn Son also bears the title “the Word.” (John 1:14) Why? Because before being born as a human, he served in heaven as one who spoke for God.
“The Word” was with Jehovah God “in the beginning,” when “the heavens and the earth” were created. He was the one to whom God said: “Let us make man in our image.” (John 1:1; Genesis 1:1, 26) Jehovah’s firstborn Son was there at his Father’s side, actively working with him. At Proverbs 8:22-31, he is represented as saying: “I came to be beside [the Creator] as a master worker, and I came to be the one he was specially fond of day by day, I being glad before him all the time.” https://wol.jw.org/en/wol/d/r1/lp-e/2005681
Jehovah's Witnesses view Jesus as the instrument of Jehovah God, the Creator:
Not a co-Creator. The Son’s share in the creative works, however, did not make him a co-Creator with his Father. The power for creation came from God through his holy spirit, or active force. (Ge 1:2; Ps 33:6) And since Jehovah is the Source of all life, all animate creation, visible and invisible, owes its life to him. (Ps 36:9) Rather than a co-Creator, then, the Son was the agent or instrumentality through whom Jehovah, the Creator, worked. Jesus himself credited God with the creation, as do all the Scriptures.—Mt 19:4-6; see CREATION. Source: Insight on the Scriptures volume 2, Jesus Christ - https://wol.jw.org/en/wol/d/r1/lp-e/1200002451?p=par#h=7
Because they see Jesus as a created being, they have added the expressions “by means of him” and “all other things” and “created through him” into the text of Colossians 1:15-16.
It is my understanding that the Christian Greek Scriptures of the New World Translation is based on the Westcott & Hort Greek text. The Kingdom Interlinear Translation of the Greek Scriptures acknowledges the fact that the NWT of the Christian Greek Scriptures is “a translation of the Westcott and Hort Greek Text, first published by them in the year 1881.”
Dr. Hort  had leanings towards Unitarianism as is made very clear by his associations and his avid support for Dr Vance Smith, the renowned Unitarian scholar on the Revision Committee of 1881. 
I am unaware of any other source that would justify the additions made to Colossians 1:15-16 that appear in the New World Translation.
 With regard to the Revision Committee, there were two opposite schools of biblical criticism: on the one side Drs Westcott and Hort, and on the other Dr Scrivener. https://books.google.co.uk/books?id=bwkIAwAAQBAJ&pg=PA502&lpg=PA502&dq=Dr+Hort+Dr+Vance&source=bl&ots=tNaIqD89lF&sig=ACfU3U0PgmTidS1zBS5XSwe-MFpPpLrUUw&hl=en&sa=X&ved=2ahUKEwjX0Lufw9boAhUSY8AKHbdFCCgQ6AEwA3oECAwQNw#v=onepage&q=Dr%20Hort%20Dr%20Vance&f=false
 Dr Vance Smith, Unitarian scholar was (controversially) invited to join the revisers.
Here is an extract from The Watchtower—1970 April 15 page 255 – Questions from Readers:
Bible writers often took for granted that certain things would be understood, just as writers in our day do. For example, the apostle Paul states, as we read at Colossians 1:16, that by means of Jesus Christ all things were created in the heavens and on earth. But since we know from Revelation 3:14 that Jesus himself was also created, the New World Translation adds the word “other,” which clearly is what the apostle had in mind. But even here, it might be added, that, were it not for the prevalence of the trinitarian teaching that Jesus was not created, it would not have been necessary to add the word “other.”
That explains why the New World Translation has added the word "other" in Colossians 1:15-17.