I heard that the way you can do this is by rearranging the way they appear in your "default" unit list. The way you can do that is to have cancelled trades; send the monsters over to an ally/friend, then cancel them. The monsters will return to your inbox, then you can choose to receive them in a different order, which will set their ordering in your defense. It's really roundabout, and hopefully they add a feature to allow rearranging in the future.
I've never actually tried this out (doesn't bother me that much if someone robs me of a few thousand Luna if I don't keep too much on me) - but maybe you'll be able to try it and see if it works.