Getting user attributes from a generic LDAP (wherever it is) is not a big problem. You need to configure a generic LDAP UDC. Follow the instructions here: http://help.qlik.com/en-US/sense/September2017/Subsystems/ManagementConsole/Content/user-directory-connectors-generic-LDAP-properties.htm
Authenticate those users against they LDAP is a different kind of job. You need to define a new virtual proxy and define a new custom authentication, because you can't use a standard windows authentication.
There is an extension on branch which can help