What do I do with useful information on a post unrelated to the question? This comes about sometimes from knowledgable users that give some background to bigger problem that solves the direct problem indirectly... Any suggestions?
Many times, users ask the wrong questions. They have a problem, and a partial solution, and ask How do I finish this partial solution?, instead of asking How do I solve my problem?
In these cases, I always answer to the best of my knowledge and never hesitate to point out other solutions that might be better than what the user wanted to do, if there's enough indication that he's asking the wrong question -- even more so when it becomes obvious the user isn't tied to a particular approach and just happened to start going in that direction. If you're not sure, you can always ask the user to explain the actual problem in a comment (which is my approach to troubleshooting questions, but with mixed results, reputation-wise, to be honest). Remember to only answer if you're reasonably sure that your answer helps the user with his problem. Somewhat related, but not really necessary, bonus information can be added in a comment on either the question or a suitable answer.
If there's some specific issue that is on-topic and you learn about it, consider searching the site for related questions. There are tons of questions with incomplete or insufficient answers, or no answers at all. This site is part Wiki for a reason: Everything can be edited, and even users below a few thousand reputation points can suggest edits for approval by others and thereby increase answer (and question) quality.
If there's no question (yet) you'd know the answer to, you can always post a question yourself, and then answer it. You won't be getting reputation when you accept your own answer, but it will improve the content on this site, helping others, and can even lead to unexpected, even better answers than you originally anticipated. I experienced this myself a few days ago.
If a comment points out that your answer is incomplete or partially wrong, you can always edit it. Make sure to not add Edit: I found out the previous three paragraphs are totally wrong, here's how it actually works: [...], but just change it however needed to optimize for future readers that have not yet seen your answer. The history of the post is preserved in any case, so there's no need to explicitly be honest or open about what you originally wrote.
As KronoS said, you probably want to give us a more concrete example.
However, if you have useful information to add to a question on Super User, which also happens to solve the problem (even if indirectly), post an answer.
Even if a question already has an accepted answer, that doesn't mean you can't add a new one. Having as many solutions as possible for any problem is the ultimate goal. We love uncommon workarounds – as long as they're helpful in some way.
If you have information that is not directly related to the question and wouldn't really solve the problem at hand, you should probably not answer. Maybe you can add a comment to the question, explain a bit, link to further resources. Hey, you could even blog about it and then use this as a reference!