That is my time consuming, programmer's workaround: http://www.question2answer.org/qa/25824/convert-comment-to-answer-mysql-for-now
I guess, it would be helpful for many users to have the option (similar to the answer edit), to convert a comment into an answer.
If you want this feature, please vote for this post so that gidgreen (the main q2a developer) will notice.
NOTE: I know that it could be problematic to decide if the following comments belong to the answer or to the other comments above. My suggestion: Instead of moving the comment to the answer, copy the comment to a new answer. The other comment can maintain or can be deleted manually then.