thanx testuser, i've pu the pieces together from your addFtpUser-script and created a shell script that does everything from the commandline.
why is it necessary (may be only on my machine???) that permissions for the homedirectory are worldreadable?
here it is:
#!/bin/sh
uName="ftp"
ftpUserDir="/Volumes/path/to/volume"
listID=`niutil -list . /users uid | awk '{print $2}' | sort -n | tail -n 1`
nextUID=`echo "$listID" + 1 | bc`
sudo niutil -create . /users/$uName
sudo niutil -createprop . /users/$uName uid $nextUID
sudo niutil -createprop . /users/$uName gid -1
sudo niutil -createprop . /users/$uName shell /bin/tcsh
sudo niutil -createprop . /users/$uName home "${ftpUserDir}/$uName"
sudo niutil -createprop . /users/$uName passwd ""
# Create ftp directory
sudo mkdir -p "${ftpUserDir}/$uName"
sudo chown ${uName}:staff "${ftpUserDir}/$uName"
"${ftpUserDir}/$uName"
sudo chmod 333 "${ftpUserDir}/$uName"
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ps: a drop box is actually what i had in mind