We don't have a solution for this yet.
Do you want this interface within BigBlueButton, or the ability to implement it in your own front-end where the user chooses from their DropBox account (you would use the DropBox API to collect the available documents), and when the user selects a file, have an API call, such as 'uploadPresentation' where you could have the presentation uploaded to the meeting.
You would probably need to download the presentation first (your application would have authenticted with DropBox), and then do a POST request using 'uploadPresentation' to send the presentation to the meeting.
Regards,... Fred