I have requested this in a Feature Request on the m0n0wall forum

http://forum.m0n0.ch/index.php/topic,5408.0.html

I hope it gets in eventually.

In the meantime there are some automated tools that will do this for you 
from a machine behind your m0n0wall.

On a Windows machine, I run this freeware

http://www.iannet.org/apps/TunnelBrokerUpdate/

For some *nix shell code this Google link points to some.

http://www.google.com/#sclient=psy-ab&hl=en&source=hp&q=tunnel%20broker%20update%20script&pbx=1&oq=&aq=&aqi=&aql=&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=588b3e7678b699e9&biw=1355&bih=848&pf=p&pdl=3000

Good luck.

FG