Unless you're going to use VLAN tagging (which means that you'll have to buy a VLAN capable switch - almost definitely more expensive than an additional network interface for your laptop), having only one physical network interface kind of defeats the purpose of having a firewall in the first place. Now of course if you want to use it for the shaping only, that's a different story, but m0n0wall wasn't designed to be used only as a traffic shaper.
There are USB Ethernet adapters out there for a reasonable price; however, before you buy one, make sure that it appears on this list:
http://www.freebsd.org/releases/6.3R/hardware-i386.html#ETHERNET