In the positive case, where you like the comment:
The current method for doing what you propose is to add a comment
"@SomeCommenter Please add your comment as an answer so I can mark it as correct"
If the commenter chooses not to create an answer after however long you're willing to wait, you could then just add an answer that contains their content.
As (link)SomeCommenter(/link) point out in the comments "The solution is X Y Z, you are incorrectly applying A B C. Also, F G I. Heh, J/K"
Make sure that the answer is marked as Community Wiki, because you do not want to look like you are trying to steal/gain rep for someone else' work. Then after 24 hours, you can mark your own answer as correct.
Obviously, if that poster chooses to add their information as a proper answer later, you should give them the checkmark and upvote, and delete your CW answer.
Negative case, where you dislike the comment:
If a user is unsure about the information they are posting, or they do not have time to adequately verify the information, then absolutely it should be a comment and not an answer. In a perfect world, no one would post anything without verifying it top to bottom and inside out, but if we don't have time to test every nook and cranny 100% in our real jobs, why would we have time to do it here? It's simply an unreasonable standard.
Now, as George Stocker mentioned, if you simply want to mark their comment as an answer so you can vote it down, that seems like a remarkably negative attitude. You are literally saying, "I dislike this comment so much that I want to be able to highlight it as a paragon of inaccuracy and let them be downvoted into oblivion". Which is not very charitable.
In the general case,
This feature is also something that could be very much abused. When users post fairly regular are low level questions, they will often get comments like, "Have you heard of goooogle??" and that comment will receive many upvotes. Although this comment is rude and not appropriate, at least it is a comment and not an answer (so the commenter got that much right). If this feature is implemented, I can guarantee there would be enough people marking it as "should be an answer" and then upvoting it. This user would be getting upvotes against their will. But I would also downvote it, or flag it, because it should not be an answer at all.
So suddenly, any comments have the ability to be turned into answers, and it depends on community will. The community will is notoriously fickle, and this will tend to screw with people's reputations, and they will have no idea why.
Lastly, and most importantly, I dash off comments whenever I think I have something useful to say, but when I answer something, I like to put time care and effort into it. I actively don't want any of my comments turned into answers, because if I had wanted that, I would have created them as answers.