The underlying Netwitness servers are running CentOS, either 6 or 7. There are many How To documents for setting up CentOS servers to use LDAP authentication for SSH. I highly suggest looking for one that best suits what you are looking for. I don't provide any recommendations here as we only have instructions for setting up two factor authentication with SecurID for accessing the servers directly. These instructions would not help in this situation.
Please remember that when you use any suggestions or how to documents that are not from RSA you must decide if the instructions pose a security risk to your environment before proceeding. As this is a security product it is important that you keep it as protected as you can. So any modifications that are not from RSA you use at your own risk. If you have any other questions please reach out to the forums, or open a customer support case, and we be glad to help you where we can.