For example, I want to post an answer of the question: "how to read from a file, skip header, plot it and fit it to a curve?" The answer can already be found as separate answers in this forum. However, as a whole, it will take me some time to look up. So is it legitimate to post this sort of questions and answer them for my convenience?
You can always post any question and answer it yourself. But both the question and its answer will judged like any other. Should the question be found too localized to your private needs, it will be closed and maybe down-voted as well. Should your answer be trivial or incomplete, it will be ignored. Should it be useless or just plain wrong, it will be down-voted.
On the other hand, should your question be posed so it raises an issue of general interest and your answer give a solution useful to future users of this site who come to it with a similar problem, then both question and answer will not only be welcome, but will earn a goodly number of reputation points.
So if you make the effort to write both question and answer with the high quality as the high-vote questions and answers you find here, your contribution will be very welcome.