We use several Moxa com servers at work to hook up serial devices to our LAN, but afaik there's no way turning it around, i.e. using the serial port to access the network since the serial device doesn't know anything about Ethernet/IP.
In theory it would be possible to build something like that of course, but I doubt there's such a device available.
With the available com servers it should be quite possible to make an Ethernet->com server->serial connection from a PC/Mac to the Amiga (requires dial-in service) and then share the PCs internet connection through that. Although it doesn't make much sense, as it's easier and much cheaper to use a serial null modem cable...
Pocket NICs are something completely different. They'd 'just' require a driver but are specifically made for PC printer ports which are quite different, so some hardware changes may be required. I haven't seen them for a number of years - might make it hard to get documentation or manufacturer's support.
