You can use any tool which let you modify the LDAP, like command line tool ldapmodify.
Or a web interface like Easy LDAP management | LDAP Account Manager (see also the howto for NethServer)
The identity must be set on the client, there is no other way (at least I don’t know it).
If there is better solution, I bet @davidep or @filippo_carletti will drop it here 