here is an article on integrating OSX into a NIS network.
i am looking through the lookupd man page. i think you do something like this: either make a NetInfo directory /locations/lookupd/users, and put a key in it LookupOrder, with the names of the lookup agents. important to include NISAgent. Or, you put a file at /etc/lookupd/users, and put those data in that file.
the full details are in the man page.
cheers