I would love that as well. I know it is possible to configure LDAP in main.inc.php but when I do this for my Memotoo account it is "Loading..." for a few minutes but never shows any contacts from the address book.
I am using the same LDAP settings in Thunderbird where everything works perfectly.
(I have not tried any other LDAP server with RoundCube because I do not know any other)
Can anyone help with this?