If you’re using the AudioProcessorValueTreeState class then you can add child ValueTree objects to the main ValueTree object inside the AudioProcessorValueTreeState object. Then this can be serialised/deserialised to/from XML as shown in the tutorial. But as these nodes won’t be associated with any parameters they won’t be automable.
Have a look at how the AudioProcessorValueTreeState::getOrCreateChildValueTree() method adds children for the parameters. This is a private method but you can write something similar to add your own nodes.