Wikipedia says: Since Classical times, Nimrod has traditionally been considered the leader of those who built the Tower of Babel in the land of Shinar, though the Bible never actually states this.
Judaic interpreters as early as Philo and Yochanan ben Zakai (1st century AD) interpreted "a mighty hunter before the Lord" (lit. "in the face of the Lord") as signifying "in opposition to the Lord"; a similar interpretation is found in Pseudo-Philo, as well as later in Symmachus.
This is how the JW's translate that scripture too, but, BOTR you are right, the Bible itself does not actually say it, it is derived from extra-biblical tradition, and "in opposition to the Lord" is not a literal translation of the text at all. It is interesting how they accept certain non-biblical traditions at the same time as rejecting anything that does not agree with their narrow point of view.
I remember that they also changed the translation to show that Noah preached about the end but the people 'took no note' when in fact the original manuscripts say no such thing.
Matthew 24:38 For as they were in those days before the flood, eating and drinking, men marrying and women being given in marriage, until the day that Noah entered into the ark; 39 and they took no note until the flood came and swept them all away, so the presence of the Son of man will be. - NWT
----All other Bibles say, in one way or another say:
Matt 24:38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39 and they knew nothing about what would happen until the flood came and took them all away.
For more proof the NWT changed the scripture to suggest there was a modern parallel with their own preaching activity:
24:39 κα? "and" ο?κ "not" ?γνωσαν "they-had-acquainted" ?ως "unto-if-which" ?λθεν "it-had-came," ? "the-one" κατακλυσμ?ς "a-dashing-down-of," κα? "and" ?ρεν "it-lifted" ?παντας, "to-along-all," ο?τως "unto-the-one-this" ?σται "it-shall-be" ? "the-one" παρουσ?α "a-being-beside-unto" το? "of-the-one" υ?ο? "of-a-Son" το? "of-the-one" ?νθρ?που. "of-a-mankind." - Westcott & Hort GNT - Literal Translation
So I agree, BOTR, not only do JW's often not have a clue what the Bible actually says but what they think they know is often based on corrupt mistranslation.