If the multitude of available options doesn’t address your individual use case, you can always write your own app using the Box API. The Box API allows you to add files and folders programmatically.
Figure 1: See http://pastebin.com/Lqu89vLM for a code sample.
There are numerous 3rd party tools such as Backup Box that allow you to move files from FTP, WebDAV, SFTP, Dropbox and Google Drive to Box.
Figure 2: Backup Box offers a free trial and works well for end user led migrations.
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