How to get underlying TCP socket from BSD socket?

Hello.

I am adding TCP-Keepalives to an existing project using BSD sockets. It seems that setting the TCP connection/idle timeout is only possible for TCP API Sockets using netTCP_SetOption() but not for BSD Sockets. (Other than the default setting in Net_Config_TCP.h.)

Since BSD Sockets are implemented on top of the UDP and TCP Socket APIs, (how) can I get the underlying native TCP connection socket from a BSD connection socket?

From Can I use both TCP native sockets and BSD sockets? I glean that I should not use "mixed" native TCP and BSD API functions on the same connection. So I would just like to set the timeout and then forget about the TCP native Socket. - Would that be fine?