It seems to me if you just enter random characters into the line settings (i.e. so it's not actually registering to a VoIP server), and then you dial something like sip:mynam@myvoipaddress.com or 500@192.168.0.30 it should dial directly to the other PC. I'm not sure why it requires you to enter VoIP account information when it's not actually required.
Alternatively you can sign-up with a free PC to PC VoIP service that offers free in-network calling between PCs and then set-up each PC's Express Talk to use an account with that service.