NIS is a way to obtain data (authentication data, path to home directory, …) about users (and other stuff). _Doing_ things (e.g. creating a directory) is not its job. So I think you're looking in the wrong place.
Most unices use PAM to manage logins (not just authentication but also credential management, account management and most relevantly session management). The Linux implementation (and others) includes a module called `pam_mkhomedir` which does exactly what you want. Put this line in `/etc/pam.d/*` for each service that allows local users to log in (or in `/etc/pam.d/common-session` if you have that):
session required pam_mkhomedir.so skel=/etc/skel/