You dont need to add the port to /etc/services, simply
# cat /etc/xinetd.d/fw-port12345
service fw-port12345
{
type = UNLISTED
port = 12345
flags = REUSE
socket_type = stream
wait = no
user = nobody
redirect = 212.122.1.3 25
per_source = UNLIMITED
disable = no
#only_from = 192.168.0.0 192.168.1.0
}