Doubtful you'd be able to do that. Adding the extender to the Hubitat Z-Wave network would only be a part of it. You would still need to write a driver in Hubitat for it to work the way you want it to.
That's how it works with SmartThings.
The other thing you can do is keep your Harmony on SmartThings and use HubConnect ([RELEASE] HubConnect - Share Devices across Multiple Hubs (even SmartThings!)) to connect your HE hub with your ST hub.
