Controlling ports used by natd

[Brett Glass]

Is there a way to control the range of ports to which FreeBSD's
natd maps outgoing connections? I'm attempting to deal with a
situation in which natd is (sometimes) changing outgoing UDP 
packets' source port numbers to ones which are commonly used 
by worms. Sometimes, a firewall at the destination blocks the
packet; at other times, the response is blocked on the way
back.

If it is possible to tell natd to avoid using ports that are
firewalled, it ought to be possible to avoid this problem. But
I can find no way to do this. Does one exist?

--Brett Glass