It's not possible to make outbound FTP requests from Google Apps Script (there is a similar restriction from App Engine as well).
You are options are either to find an HTTP interface to the server (some sites offer both HTTP and FTP), or download the files through an FTP client and then upload to Google Drive and then access those files from there.
Feel free to look for/log an item in the Issue Tracker.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…