DHCP options are stored, at the end, in /etc/dhcp3/dchpd.conf
This is not available via Zentyal GUI so the only way is to modify *.mas file or better to use hooks.
The idea is to add, in the right subnet "option 181 PNAgent-IP-address" but better, if not ideal, would be GUI allowing such kind of add-on:
DHCP option number; value using 2 fields without any control as this can not be done easily unless you store somewhere what each and every option should contain (string, number etc..)