I see some great answers in comments. complete well written on topic or maybe an edit or two, but answers, not comments.
Comments are (by design) limited in length; while it's possible to provide something of an answer in the space of a comment, it's better for answers to be more fleshed-out. Sometimes you can actually do that in 500 characters, but in general, comments converted straight into answers probably won't be very useful answers. If you see a comment that has the beginnings of a good answer -- or a complete one for that matter -- please feel free to comment back to that person and ask that it be worked into an answer. We've seen that happen to good effect here.
I would be reluctant to create answers on behalf of other users; it feels kind of like putting words into his mouth. Even if they're his words, he chose to comment when he could have answered, so he may have had a reason. If the reason was "oops, didn't think about it" then the approach above will produce the desired result, but if he had a different reason, we shouldn't override his decision. Answers are more visible and more permanent than comments and they affect reputation; creating them should be the author's choice.