I don't think this is possible with NTA. There is an interesting article at this link which was written by a fellow Thwack member.
You do have some options and most use network packets as a data source. You just need to setup a SPAN or mirror port and applications like Dropbox can be detected by looking at the packet payloads. Once this is in place you can either use the QoE features in NPM 11.x or use a product like LANGuardian to grab metadata from the network packets which is used to identify cloud services like DropBox which may be riding on TCP port 80 to various AWS addresses.The LANGuardian can be integrated with Orion so you retain a single pane of glass view of your network activity. Demo at this link
http://demo2.netfort.com/Orion/SummaryView.aspx?viewid=31&AccountID=guest
Hope this helps,
Darragh
