You may be able to get by with a RS-232 to Ethernet convert, then buy a ethernet switch to connect all the cabling. The only problem I see would be that you'll need multiple RS-232 ports on one single computer for your scenario to work as you want.
Here is a link to a RS-232/Ethernet converter that may help http://www.icsdatacom.com/ethernet_RS232_ds.html
Also I stumbled across this as well, it looks like just what you need as well. http://www.nti1.ca/serial-ip.html
Would help very much in connecting those CNC machines to a standard Ethernet TCP/IP network.