This might be old and superseded but I have a number of devices only visible in Home Assistant e.g. my Zigbee lighting, that I wish to control using Alexa voice commands, and tried this that seems to work for relatively simple scenarios such as on/off and few brightness levels.
I simply put Tasmota on a D1 ESP8266 dev board and configured it as a My_Test_Dimmer with no actual connections, other than USB for one-time flashing, or extra components. It is just as it comes out of the packet. Hue bridge and MQTT were enabled too. I used a simple Tasmota configuration file from Search Devices Supported by Tasmota.
I then set up several simple Alexa Routines that will ultimately be used to switch devices on/off or whatever setting is required. In the routine, I enter the required phrase and select the My_Test_Dimmer device, setting brightness to some arbitrary value such as 50%. I then create automation in HA that use the dimmer’s MQTT status message containing the brightness value (50% in this case) as a trigger to do something such as turn a light on that Alexa has no visibility of. It may not be very efficient but it seems a simple way to potentially control many device settings. Have I missed a simpler method or is this flawed in some way? I will elaborate on this method if requested or be redirected to existing similar better posts.
There have been a few other methods of doing the same thing with no need of hardware at all by creating a virtual dimmable light in HA.
you can do a search for “dummy light” to bring up some examples but here is one:
and one that takes the concept to a whole new level of functionality (and complexity! 
):
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