To create a local directory, I've been using this trick for years:
/tool fetch address=127.0.0.1 src-path=someDummyFile user=admin password=xxxx \
The fetch command seems to set up the credentials with ppp and then ftp creates the newDirName. Just delete the newFileName if you don't need it, or set its contents if that's what you want to do. Since I'm creating the folder on the local file system, I use the loop-back address; I have not had need to try a remote IP.
I've recently discovered a serious problem with this however:
After upgrading from v5.4 to v5.21, I found that a system reset-configuration run-after-reset
script--which by necessity runs as user sys
--is refused FTP admin login: If I run this from reset-config in 5.21, I get a "530 Login incorrect" error and the script aborts. But if you run the script while logged in, that shouldn't be a problem.