You can add an entry to the directory using ldapadd(1). You can specify a single entry on the command line, or you can specify one or more entries in a file. See the SIMS Reference Manual or the ldapadd(1) manpage for details of how to use ldapadd.
You can use ldapsync to read information from the /etc/passwd file or /etc/aliases file and create a file suitable for use with ldapadd.