Assuming you have no root/admin access to the source LDAP server, you could use LDAP searches for "replicating" the data.
But please be aware of the fact that LDAP servers usually come with ACLs which means that you might not have access to all the data.
Furthermore, LDAP searches won't give you the "hidden" or system attributes, such as modificationDate, entryCSN etc.
Use the following command to have a look at the source LDAP server:
ldapsearch -x -LLL -h <IP of LDAP server>
If this command doesn't work, you most probably do not have your ldap.conf configured with the credentials. If you have done that and there is still no output, use filters etc., such as:
ldapsearch -x -LLL cn=*
ldapsearch -x -LLL uid=*
If you are unsure what to look for, I highly recommend to connect to the LDAP server with one of the following tools:
These tools allow LDAP browsing and will give you an overview about the source LDAP structure.
When running a few LDAP searches and writing the output to a ldif, you can therefore "replicate" the data.