The two examples you give do not show either the addition of words to clarify the meaning, or adding words (for other reasons). Let me explain by showing you how a literal translation of the Greek text reads, working from an Interlinear of the Nestle Greek Text.
John 18:20 – “Answered him Jesus: I with plainness have spoken to the world; I always taught in a synagogue and in the temple, where all the Jews come together, and in secret I spoke nothing.”
This shows that no words have been added, because the Greek text itself has, ‘where all the Jews come together’. Actually, the word ‘the’ has been dropped, unnecessarily.
John 18:25 – “Now was Simon Peter standing and warming himself. They said therefore to him: Not also thou of the disciples of him art? Denied that one and said: I am not.”
You question whether rendering this ‘are you’ is an example of epexegesis, turning a statement into a question. However, the literal Greek is clearly a question. Peter is not being told he is a disciple of Jesus. He is being asked to confirm the suspicion. It’s just differences between koine Greek and modern English that might make you wonder if it was a statement and not a question. There is no question but that it was a question!
Someone else commented as an example of eisegesis, Luther’s translation of Romans 5:1-2 having the word ‘alone’ added, when it just is not in the Greek text. The text states, “Having been justified therefore by faith…” and the word ‘alone’ is not next to it, or anywhere in the passage. This is, indeed, an example of adding a word to make a particular emphasis. Here is what a scholarly book says about this:
“These presuppositions affected [his] translation but slightly. Yet
occasionally an overly Pauline turn is discernible. There is the
famous example where Luther rendered “justification by faith” as
“justification by faith alone.” When taken to task for this liberty,
he replied that he was not translating words but ideas, and that the
extra word was necessary in German in order to bring out the force of
the original. Throughout all the revisions of his lifetime he would
never relinquish that word ‘alone’.” Here I Stand - Martin Luther by
Roland Bainton p332, Lion 1998
What Luther did there at that one point was exactly what many modern translators of the Bible do when they seek to give a dynamic equivalence rendering, sometimes called functional equivalence, or thought-for-thought translation. This is different to older translation such as the A.V. which gives formal equivalence, literal, word-for-word renditions.
Unfortunately, this new trend to claim to clarify the meaning has led to wrong doctrine being smuggled into the translation, and this is where epexegetical examples can be given. The New World Translation of the Christian Greek Scriptures in its 1969 Interlinear version (the Kingdom Interlinear Translation) added words that were never in the text to try to get away from the biblical teaching that all Christians are indwelt with the Holy Spirit, which is the sense meant whenever the NT uses the phrase “in Christ”. Because that doctrine is anathema to the producers of the NWT (they claim only 144,000 Christians can ever be in union with Christ by having the anointing of ‘holy spirit’) they have added the words ‘union with’ between ‘in’ and ‘Christ’. Consider first how the A.V. renders Galatians 3:28, followed by how the KIT translates it, noting how square brackets enclose the added words:
“There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.” A.V. – no added words.
“There is neither Jew nor Greek, there is neither slave nor freeman, there is neither male nor female; for you are all one [person] in [union with] Christ.” KIT - three added words.
Now, the first added word, [person] is not misleading or hinting at any different doctrine (it’s just plain unnecessary). But the second square brackets enclose [union with] which supports the Jehovah’s Witness doctrine that this uniting with Christ is not confined to only 144,000 people, because although millions of them are not anointed with the Holy Spirit, they have been told they are still in union with Christ, as in, “in harmony with Christ”. That is not what the text is even hinting at. Context shows that it is the indwelling of the Holy Spirit in all Christians that gives them this spiritual union that nothing can break.
It is not within the remit of this question to delve into that. Nor will I detail the many other times in the KIT where [union with] has been added. I simply give one example of epexegesis: words added supposedly to clarify the meaning, when actually they have been added to take away from the clear teaching of scripture, but you would have to spot the words with square brackets and then be able to compare them with the Greek text, as in an Interlinear. When you look at the Kingdom Interlinear, it is clear that ‘union with’ is not even a hint in the text, and when you know the publisher’s doctrine, you see what they are trying to do. I hope these examples clarify.