If you have not already done so it is recommended one read the My Cloud User Manual (http://www.wdc.com/wdproducts/library/?id=439&type=25) to learn about the options/features of the My Cloud.
Yes the My Cloud can be accessed via FTP. One can enabled FTP access through the My Cloud Dashboard. See the User Manual or the Dashboard Help for more information. Officially, no the My Cloud cannot access another device via FTP and copy data back to the My Cloud.
Unofficially because the My Cloud has many of the typical commands that other Linux OS’s have, it is possible to use SSH to access the My Cloud through command line access and use commands like rsync or wget to copy data from one location to another.
There is also the following threads where an FTP front end for the My Cloud was discussed that may be relevant.
https://community.wd.com/t/ftp-client-with-web-front-end/95632
