The macosxhints thread linked to was posted in 2002.
Here is the Pre-Leopard operation in a slightly more readable form.
Kirkville - Creating User Accounts from the Command Line

Which OS X version are you performing the user add on?

ni tools (niutil, nidump,..) are gone from Leopard (10.5).

dscl, dseditgroup, mkdir, cp -R, and chown would seem to be the tools for this under Leopard.