OT - Cisco router question

I bought a Cisco router off of ebay to play around with. Does anyone
know if there's a way to to use the serial connections as additional
ethernet connections? I don't plan on using the serial connections
for any type of WAN connectivity, so it'd be nice if I could get an
adapter of some sort that would allow me to use the serial ports as
additional ethernet LAN ports.
Thanks,
~M