You COULD assign a uid (unit ID) to bosses without one via editing the JS files, but not only - if I'm not mistaken - does author not approve of that, summoning the units that way spawns the "default" version of those units, whereas their boss-tier stats come from...well...being assigned as a boss by the dungeon setting. For example, Bandit Queen's default HP is 400 - your companions will make short work of her.
Short version: it's possible, risky (I haven't broken a save yet, but game failed to start plenty of times afterwards), and all you get to show for it is a very weak unit.
Poking around, however, made me wonder whether it's possible to assign multiple bosses to spawn together and/or override the default boss settings (via a mod, for example - replacing already-existing content instead of adding more of it without editing the original files, a-la Skyrim).