There several methodologies for transitioning NIS maps into LDAP.
None of them are perfect. As almost anything can be put in NIS in almost any format, it is very difficult to provide a tool that will perform a perfect transition on any NIS server implementation.
Migration Tools
#
The MigrationTools are an open-source set of Perl scripts for migrating users, groups, aliases, hosts, netgroups, networks, protocols, RPCs, and services from existing nameservices (flat files, NIS, and NetInfo) to LDAP.
If you are comfortable with perl, these maybe very useful.