Be very careful with this.
Stack Overflow is a primarily (always?) a factual site. Very rarely can a factual answer be expanded upon in a way that shouldn't simply modify an existing answer. However, it is possible to do. What you need to ensure is:
- You're actually answering the question. If you're not answering the question in a direct way, you need to revise what you're doing. If you're offering, for instance, an approach that is not what the author asked for, you probably shouldn't post another answer. If you're expanding on an answer in a way that directly pertains to the problem, it's alright.
- Your answer actually needs to be written in an answer. This implies that your answer is long, complete, and would not be better suited as a modification to an existing answer. If these conditions are met (pay attention to the last one), you're doing okay.
The conditions are slightly different on open-ended/subjective questions, which do not appear on Stack Overflow.
When answering a factual question, what you really need to ask is this: Does my answer provide a viewpoint that directly pertains to the question, and that does not exist in another answer? If you're answering a subjective/open-ended question, you need to ask: Does my answer provide a viewpoint that has not already been discussed? Am I providing anything new?
I would personally say that your answer would likely be better suited as a comment. It is a useful qualification to the answers, but it doesn't really directly answer the question.