Yes. Add it and explain in the starting paragraph that it doesn't answer the question in its entirety, so others realize there might be something to add to the whole thread and are not discouraged in posting their own answers. We're not supposed to encourage this, but a partial and still helpful answer is surely better than none. And sometimes, different answers can complement each other, especially on slightly broader questions. Go for it. ;)
And if you find any comments helpful under questions that don't have posted answers, you have my blessing (not that you need it, it's just a figure of speech) to reuse them in your answer, no matter whose they are. Comments are supposed to be transient in nature, serve to resolve issues with posts they're made under, and not answer the questions. But it does happen that sometimes their posters want to help the author of the question out, and later don't find the time for a full write-up. Don't consider any comments as territorial marks, our goal is to provide as complete as possible answer, not links to external resources or hints to what proper answers might look like. And flag any no longer needed comments as obsolete once you're done, so some moderator clears them and they don't steal attention from the Q&A.