SSL VPN with LDAP and client certificate auth

I have a working ssl vpn with ldap authentication.  I would like to add the client certificate option for added security.  I have been reading the cookbook on setting up the sslvpn with certificate auth, but it does not mention using ldap.  The directions have you create a PKI user for use with the certificate, but if you try to do this for a user that was added with ldap, it fails because they already exist.  How can I continue to use ldap auth for the sslvpn, but add user cert auth ?