Chapter 7 Firewall for FortiOS 5.0 : Multicast forwarding : Multicast routing examples : Example PIM configuration that uses BSR to find the RP : Adding the NAT multicast policy
  
Adding the NAT multicast policy
In this example, the incoming multicast policy does the address translation. The NAT address should be the same as the IP address of the of loopback interface. The DNAT address is the translated address, which should be a new group.
config firewall multicast-policy
edit 1
set dstintf port6
set srcintf lo0
next
edit 2
set dnat 238.1.1.1
set dstintf lo0
set nat 1.4.50.4
set srcintf port1
next