Antonio:
What you ask is a radius server specification. You need to look into a radius server that supports your LDAP server and that piece of configuration is done on the radius server, not on the WLC.
On the WLC what you do is configure normal radius server ip address and shared secret and the WLC will communicate with the radius server. Now, when the radius server receives the request it should be configured to do a request to the LDAP server and check the credentials on an external LDAP DB, not in local users.
HTH
Amjad
Rating useful replies is more useful than saying "Thank you"