Interesting idea: Take a device which can do conversion between TCP/IP and a Serial Port ...

http://www.toniwestbrook.com/archives/148

This is not very cheap, but seems to be an easy way to get files through an Ethernet network or even via WiFi connection from and to a CP/M computer.
Also, a modem can be emulated (think about using RBBS hosted on an old CP/M computer).
Unfortunately, this doesn't mean you have something like a drive extension (getting files from and to a PC), and so you're still limited to communication programs at the CP/M side.

May be Kermit can be used (at PC side, PC-Kermit 95 through network, at CP/M side, the old Kermit 4.11). I don't know. Have to ask Toni (I will update this entry if I get an answer).