For a simpler approach though, you might just want to create a Perl script on your PDC and run it as admin, which will dump all of your users and groups to a file. You could then write a corresponding Perl script to parse the file and turn it into an ldif file, which you could use to insert the users into your LDAP server.
Of course, obtaining the Windows passwords or password hashes is not achievable using either of these methods.
A script to convert the pwdump output to ldif can be found at <ftp://ftp.openldap.org/incoming/norbert.klasen-20001120.migrate_pwdump.pl>
Norbert