You need to have telnet access to your synology box... then:
1. install ncftp with the command "ipkg install ncftp"
2. make a script to download the files you want. I used it to download some logs from a weserver every night. With ncftp you only need a single line in the script. My script was something like this (the entire file):
"ncftpget -u ftpusername -p ftppassword ftphost destinationfolder filetodownload". You can see the complete syntax in http://www.ncftpd.com/ncftp/doc/ncftpget.html
3. Add a line to the file /etc/crontab to setup a job that downloads every night. Use a line that looks similar to:
"0 4 * * * root /volume1/scriptfilename.sh"
This line will setup a cron job that runs the scripfilename.sh script every night at 4.
And that's it....