In prefs.js; AFAIK there is no other way yet but to edit but by hand.
this is the test entry i created to show You.
user_pref("kmeleon.group.test", "
http://kmeleonbrowser.org/forum/list.php?1 ");
user_pref("kmeleon.groupnames", "\ntest\n");
btw. Maxton is a good IE shell!
mfG