You can't change the location that reseller backups are saved to. By default it should be /home/reseller/user_backups
Solution 1: pre-create the user_backups directory as a link to the directory you prefer.
Solution 2: It may be possible to save the backups to an ftp location using the loopback function (if proftpd allows this) to "ftp upload" them to any directory of your choosing.
However I am unsure of your question. Perhaps providing an example or more details would help.
Solution 2: It may be possible to save the backups to an ftp location using the loopback function (if proftpd allows this) to "ftp upload" them to any directory of your choosing.