When I get the best answer to my question in a comment, is it possible to convert this comment into an answer, and accept it? Sometimes the poster don't bother to do the conversion, which is a problem because I would like to accept the best answer.
No, it's not possible to convert a comment to an answer, and it's not really a good idea either.
If I choose to post an answer as a comment you can assume that I don't feel it stands as an answer, at least not an answer that I want posted under my name. If you disagree and feel the comment is a good enough answer, feel free to post it as an answer yourself, that's perfectly fine and even encouraged.
If you feel a comment is the answer to your question then you can ask the commentor to write one in order to accept it. Most users will do that because everyone likes a little reputation.
But if not then you could write an answer yourself. But don't just copy a comment into an answer. An answer should not just be a one-liner. It should explain the matter.
An automatism to convert comments into answers is not the way to go. That would not meet our quality standards for answers on SE.