I would suggest you take a look at Automator to do this.
Edit : If dropbox folder is on indexed drive then mdfind is fast. But like subsonix said if you want iTunes to be aware of them you need to import them.
I don't think it's enough to just move the files, if you want iTunes to be aware of them you need to import them. There is an "import to iTunes" action available in Automator though, and it's also available by default in the context menu on OS X if you select a media file.