Hey @rustlerski - Since I believe Actions are a required parameter for Commands, and Actions are only (seemingly) added to a device from a prescribed list of Actions during each device’s initial setup / addition to the Bridge, I have not had any luck adding custom Commands.
I have, however, had decent luck setting up a custom device as a normal Fireplace or Fan, keeping careful track of which Commands / Actions I mapped in the app to the physical original remote, and then PATCH-ing the Commands after finishing setup. See here.