Below the answer, there is a “more” link (menu). If you click on it you are offered Repost answer as comment. I don’t know if karma is needed.
In case you need karma for this repost, an alternate way is to open a comment, copy and paste the “answer” and post comment. Then you delete your answer.
However, be aware that a comment is limiter to ~1000 characters while an answer is virtually unlimited.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!