The development of a general-purpose library is often guided by 2 contradicting desires: the wish to make it as complete as possible, and on the other hand, the wish to keep things simple. TCP Time-stamping has disappeared from the library in order to decrease its complexity.
But it is possibly not TCP time-stamping what you want to do?
Like Richard suggested, you could add a new field to NetworkBufferDescriptor_t:
typedef struct xNETWORK_BUFFER
{
...
#if( ipconfigUSE_MESSAGE_TIME_STAMP != 0 )
uint32_t ulTimeStamp;
#endif
} NetworkBufferDescriptor_t;
Just curiosity: does your application have to do with ( synchronising ) audio?