TCP/IP is a full network protocol, serial is just a series of on/off messages on 9/15 pin wire. It's not possible to directly change one into the other.
I realised that, and also an app requiring direct pin access to a NIC's UTP port would probably require some major work, a device driver and admin privs :-) I'm not recommending that approach :-)
At worst a DirectPlay (part of DirectX) implementation might make matters easier, though that would lose WinUAE -> UAE inter-operability in this respect.