I agree with possible-san: more generally the translation of prepositions is a much bigger problem in the NWT of this passage than the addition of "other" (with or without brackets)
He is the image of the invisible God, the firstborn of all creation; 16 because by means of him all [other] things were created in the heavens and upon the earth, the things visible and the things invisible, no matter whether they are thrones or lordships or governments or authorities. All [other] things have been created through him and for him. 17 Also, he is before all [other] things and by means of him all [other] things were made to exist, 18 and he is the head of the body, the congregation. He is the beginning, the firstborn from the dead, that he might become the one who is first in all things; 19 because [God] saw good for all fullness to dwell in him, 20 and through him to reconcile again to himself all [other] things by making peace through the blood [he shed] on the torture stake, no matter whether they are the things upon the earth or the things in the heavens.
In the NWT both "by means of" and "in" stand for the same Greek preposition en. In the first instance, by means of reduces its scope to an instrumental meaning and makes it semantically equivalent to dia, which is consistently rendered as "through," making v. 16 redundant -- which is probably not the case in Greek: "In him all things were created / All things were created through him" are different and complementary ideas. The damage is even worse in v. 17 as it leads to a mistranslation of the verb: by means of him all [other] things were made to exist misses the nuance of sunistèmi (from which our word "system" derives): in him all things hold together.