Yeah, spawned skele's don't expire until they die. So at the beginning of my daily playthrough I spawn them outside my walls and they beat on anything that gets near and when I log in the next day they're still there sometimes if they haven't been killed.

I tried leveling them up in a dungeon with a spawner but they'll run in there and destroy the spawner so I'll have to lift up one of the weaker enemy spawners so they can't destroy it.