You will have to use a combination of USME to set up the monster pack, and FF6MDE to set up the chests contents, and then the Level Editor to place the chests on the map, unless you just want to use a pre-existing chests. So in FF6MDE, go to "Chests." There you can set up your newly created monster pack (the boss) to a chest. It can be pretty confusing.
We are born, live, die and then do the same thing over again.