Am I able to edit /etc/multiconf/dhcpserver and add an additional entry for the DMZ interface? Can I also edit /etc/dhcp.conf to add the subnet? I would like to put a wireless access point outside the firewall on the DMZ.

Here is what you can do:

1) Add the DMZ subnet through Web GUI in DHCP Server page.
2) Edit the /etc/init.d/dhcpd file. Search for "eth0" and modify the line "dhcpd eth0" to "dhcpd eth0 eth2"
3) /etc/init.d/dhcpd restart