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.
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.
We have written a Java application that will read a running NIS server and convert the NIS data to LDIF files
The N2L Service will move your NIS information into LDAP based on your configuration, when you initialize the N2L service. The N2L service is only supported on SUN's LDAP server, but we have sound if careful schema setup is followed it will work on with other LDAP servers.
There might be more information for this subject on one of the following: