How to translate UDP unicast packet to a broadcast address via DNAT with
iptables or something else?
With iptables is not possible (AFAIK) unless you allow the broadcass traffic to pass to the router (IMHO a bad idea).
DNAT is performed in PREROUTING, the packets are DNAT'ed to broadcast in this chain, and when these packets arrive to routing decision it's broadcast traffic wich is usually blocked and doesn't go to FORWARD.
Other option is to set an static ARP entry wich translates the target unicast IP to broadcast ethernet address.