LDAP Question

To List,

WE use LDAP authentication for RT. What we want to do is have the LDAP 

authentication code evaluate the UserID so that not only the UserID
works, but also the email address AND the Employee Id can be used as
well. I thought there might be a way to point some LDAP configuration
setting to point to a different directory/file or different parameter
passed to allow such a condition. Does anyone have any experience with
this kind of thing? Thanks in advance.