I don't know if the troop editor allows you add troops, or only modify them?
It only messes with troops.txt - which is the "compiled" version of the file. The source file is "module_troops.py", which is where "troops.txt" comes from, and where TaleWorlds makes its own changes, and where they intended for the modding community to make theirs as well. Mucking directly with "troops.txt" is obviously very powerful and do-able, but you're bypassing one step, and in doing so you're more limited in what exactly can be accomplished (though it becomes easier to do the fewer things that can be done).
So... someone else will have to answer whether a new troop can be added. However, even if it can, it would need to be hooked up - something would need to refer to it from the rest of the game - either by setting it as an upgrade path for an existing troop, or modifying the party templates, so that the given troop is created as part of creating a given type of party, etc.
In the module system (the .py code), all of these thing tie together with everything else. So you can do a ton of stuff. But in the troops editor, I suspect that its primarily intended to just modify existing troop types to your liking, or to give you a quick & dirty test bed for a new troop type (by replacing an existing one for trial purposes).