Can this be used to spawn prefab buildings like from outpost?  I have an Arena mod, and I would love to have buildings from that be part of the Arena.

If you want to use it in your plugin, yes it can generate buildings, just select its position in the configuration file and use the PluginReference method.

Use the example of the umod documentation <https://umod.org/documentation/api/integration> and call the method

SpawnEntityGroup.Call("SpawnLooter", YourVector3Position);