translation to random source port?

I have:

map ral0 from any port = 4791 to any -> 0/32 portmap tcp/udp 5000:60000

As documented in man page I see it does use incremental port numbers for 
the new port (5000, 5001, 5002, ... as seen with tcpdump.)

Anyway to randomize my new source port?

Or does "random" ports need to be coded?

Note I am interested in using this in places where I can't modify the 
original software or improve operating systems to use random source ports.

I am using IP Filter: v4.1.23 (488) on NetBSD/i386 4.99.55 (March 17, 
2008)

I see the homepage says 4.1.29 is the current version. But the "What's new 
? Click here" link (ipfil-new.html) doesn't document recent changes. If 
translation to a random source port is available, please let me know.

Or would a divert socket be an acceptable workaround for this? (I haven't 
researched what needs to be done there either.)