Brainshark's conversion process transforms PowerPoint slides to images. You are not able to extract the source file from a Brainshark presentation.
If you wish to include the original PowerPoint file, upload it directly to your presentation as an attachment. You can then decide whether you want your viewers to download the original PowerPoint file associated with the presentation, or if you simply want to house it with the presentation for safekeeping.
For more information on how to add attachments to your presentation, please view this article.