I dont know why you want to use a serial to ethernet converter, because of the difference in speeds of the two types of interfaces, the box which converts should have enough buffer to buffer packets coming in on ethernet interface at 10 Mbps and send them out onto a serial interface which is usually clocked at 1.5 Mbps (though most serial interfaces on Cisco routers are capable of banwidth upto 4 or 8 Mbps).
Check this link.
http://www.stayonline.com/serial_to_ethernet/index.asp
The converts require an RS232 interface. You could probably by a DB60 (Cisco router end) to RS-232 connector cable (There is a Cisco cable part number for this too) and probably this cable can be used to connect the serial-to-ethernet converter.
I have not tried this, so I do not guarantee that it will work.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus