I recall I didn't use the new input system so as to not force developers who download the asset to use it, so you're right built-in support is lacking.
However adding support for your case should be fairly straightforward! I provided the IEntityInputProvider interface, which the entity looks for on Awake to get input. If you create a custom component that uses the new input system and implements that interface, then replace the PlayerInput component on the player with your own input component, it should work seamlessly! The mob entity class doesn't care what class is providing the input as long as it implements the IEntityInputProvider interface. Hope this helps.