If you add this to postbit legacy you'd be tweeting single posts, not threads... but if you really want to...
In Postbit Legacy Find:
And directly below it add:
Code:
<a href="http://twitter.com/share" class="twitter-share-button" data-count="horizontal" data-url="http://www.juot.net/forums/showthread.php?$session[sessionurl]p=$post[postid]&postcount=$post[postcount]" data-via="YOUR_TWITTER_USERNAME">Tweet</a><script type="text/javascript" src="http://platform.twitter.com/widgets.js"></script>
Change http://www.juot.net/forums/ to your forum's URL, whatever would come before showpost.php... and change YOUR_TWITTER_USERNAME to your twitter username... this will put the tweet button right next to the Quote button and it lines up well, I tested it. It also sends the right URL to twitter.