I posted a question about a CPU transistor and finally a member has answered it, but in the comment section. I asked him to put that in an answer so that every one can benefit from the information, but he/she didn't reply. Can I screenshot his/her comment with his/her pseudo and put it in the answer?
In general what you're proposing is OK but don't do it as a screenshot, instead copy the comment(s) as text to the answer and give credit to the person that wrote it with a link to their profile. It's always better to post anything that can be text that way so that search engines index it.
I'm not sure exactly which comment you're referring to, but it's also worth checking it's a fairly complete and correct answer to the question. Sometimes people post partial answers as comments because they don't feel it's complete enough to post as an answer