I had created a bounty for this question. All the answers I've tried and none worked. Also, the bounty expired. Today, I found the answer on IRC and I would really like to award this. So, is this possible?
Once the bounty period ends, you cannot assign it anymore. Since you are saying that none of the answers helped you, I guess that it is one of the following cases:
A user is going to write the answer, or a user already wrote the answer that helped you, but you are late to assign the bounty
In this case, you need to create another bounty that you then assign to the answer that helps you.
You are going to answer your own question basing on what you discovered
You cannot assign the bounty to yourself, so it is pointless to create a new bounty for that question
In any case, you can assign the bounty just to an answer, not to a user. This means that, if a user helped you in someway, but that user doesn't write an answer, you cannot assign a bounty to that user.