At the present time, from the section [Router]->[Virtual Server] you can create single TCP/UDP port forwarding.
In any case, if you know the syntax of the iptables comand you could forward a range of port.
I think the better place in which you should create the DNAT is the PREROUTING chain because it is not manipulated by the Firewall GUI of Zeroshell.
In the near future, I will enable the port range forwarding from the web gui interface too.
Regards
Fulvio