EDIT: Re-reading the thread, I see that this is not what you're looking for.

If you're admin on that box, then you can avoid password prompt by using NOPASSWD in the sudoers file. Like this:

# User privilege specification
root    ALL=(ALL) ALL
ftpauth ALL=(ftpdata) NOPASSWD: /bin/mkdir
[download]

Which will allow the user ftpauth to run mkdir as user ftpdata without asking for password.