Title pretty much says it all.
At this moment we managed to successfully create a test connector using GraphDB Elastic Search connector. We want to use Elastic Search UI in our React.js project. For this we need to create an engine in Elastic App Search and import documents from our connector.
We currently have a index from GraphDB currently avaliable in Kibana index management section
Like I said, our goal right now is to create a test engine using this index from GraphDB connector, but there's no way to import this data. 
I don't think adding an existing index is an option. See this discussion on Elastic Forum https://discuss.elastic.co/t/can-you-hook-an-existing-elasticsearch-index-to-elastic-app-search/222508