how can one integrate third party spice simulator like ngspice into cadence IC610?

For details on interfacing a simulator into the Virtuoso Analog Design Environment, you should contact Cadence. You'd need the "OASIS" product, but also you'd need to request the documentation and support for doing such an integration. Because it's not an everyday occurrence, Cadence tends to have a dedicated channel for support requests when doing such an integration.

I'd suggest starting by contacting http://support.cadence.com

Regards,

Andrew.