There was a set of scripts that worked much the way useradd works on linux...

They actually took the values you spec'd and entered them into netinfo for you.

go here:
http://www.osxgnu.org/
and look for the OSX user utils.

Once installed, and figured out, they should be what you are looking for... the author actually responded to an email of mine, in a timely manner, some 6 to 8 months ago.

One thing to look out for is that the useradd script doesn't actually create the user folders. just creates the account (if I recall correctly.)
That's all I know...