Regarding Adding reference data to my stream in Microsoft fabric event stream.

Makes sense. In this case, the pattern is established. Eventstream loads the initial stream into Eventhouse, where the stream is contextualized with the reference data and written to a new table. Then create a new eventstream that uses an Azure Data Explorer source (with the Eventhouse uri information). Even though the source says ADX Eventhouse is the same engine. 

Then add a custom endpoint destination where your customer can subscribe. THe other thing you could consider is rather streaming back into the eventstream simply make the data available in eventhouse via the rest api