Feature to add custom attributes to existing entities

Based on the responses(or lack thereof) on a question I asked in another category, there isn’t a way to add custom data to an existing entity. In my particular example I have a Z-wave plug bridged through a Veralite that reports total power usage. The vera does the math to convert that into a “realtime” usage in watts and I’d like to be able to see that on the representation in Home Assistant. The best workaround I can think of is to create a REST sensor and send all of the existing data plus my own that I’ve calculated but this feels very hacky and I’d like to avoid it in the long term.

In my mind the way to do this would be an update to, for instance, states.sensor.livingroom.attributes[“new attribute”]. Once a non-existent attribute gets an update call it is created and remains on the entity.