I think passwd's path recently got changed to /bin and it seems like superadduser's script isn't aware of that. Should be a simple fix, just change /usr/bin/passwd to /bin/passwd or have a check for that. Below is the output I encountered.
Enter the new value, or press ENTER for the default
Full Name :
Room Number :
Work Phone :
Home Phone :
/usr/sbin/superadduser: line 359: /usr/bin/passwd: No such file or directory
* WARNING: An error occured while setting the password for
this account. Please manually investigate this *
oops, you're correct.
actually, it's not needed as spanky now installs a symlink in /usr/bin to the real binary in ../../bin (remerge shadow after sync'ing).