slapd 2.5 forwarding request .. please help

Hi everyone, I have a particular setup and I need your help.

I have a Slapd 2.5 running in a debian virtual machine. This ldap server has its own tree, dc=example,dc=com.

I need this slapd server to forward the authentication requests for two domains other than its own, for example

dc=example2,dc=com towards -> 192.168.1.1 dc=example3,dc=com towards -> 192.168.2.1

I read in the documentation that it is possible to do this forward but I can't modify the configuration using ldapmodify.

Can you help me ? Thanks Willy