How do I add more information to an answer which is selected as best answer?

You can't. Once a BA has been chosen, the question is locked.

If the asker's profile shows an e-mail or an IM you can chat with them that way. Otherwise, no.

There are two ways.

1. Use the comment section under the question when you look at it. Note that nobody is notified so your additions will probably not be seen.

2. Email or IM the person through their profile. If they don't allow it, respect their privacy.

Those are the only two choices.

Go to their profile and see if they allow emails so you can talk. If not then go to the question and under it where it says comment write what you want to add.

Two options, under the question to the right, you'll be able to add comments. Or, click on their pic or name to see their profile. They may permit emails. If they do, make sure that your email is set to allow others to respond.

Why two thumbs down to a perfectly correct answer?

Here is a resolved question: http://uk.answers.yahoo.com/question/index;_ylt=Ag...

Now, look under the best answer and you'll see a comment box. Click it, use it, then stick your thumbs up assholes.