Is there a usermod equivalent in terminal for OS X 10.6.1

dscl is the usual way to modify any stuff from Directory Services (whether local or remote) from the command line. See the dscl manpage for all the info (or the local one if you are not running 10.6). Google turns up many examples of how to use it (including many from macosxhints.com).

"To add an example, this is how you can modify someones homedirectory:

dscl . -create /Users/postgres NFSHomeDirectory /opt/local/var/db/postgresql84

You can see also properties for an object using the read command:

dscl . -read /Users/postgres"