Came here looking for the same thing. It would be great if you can point app search to an elastic search cluster and specify an index to use. It looks like the front end and API credentials of APP Search will cut down on some of our development work.
If you're running self hosted app search, like I am, with a docker compose, I would add Kibana to the docker compose and expose the ports for ES. Than look at how the indexes are organised from App search. Than reindex from the original es cluster to the app search es cluster. Test and push to production.
Haven't had a chance to try it yet, but should work in theory, I'm assuming an 'Engine' in app search maps 1:1 with an elastic search index.
Note: I haven't attempted this, I've added evaluate "app search" to our backlog, but it could be weeks or more before we get there. I'll try to remember to update this with a better solution if no one offers one first.
It should be easy to use logstashs "elasticsearch input plugin" + "App search output" plugin in order to read from an existing elasticsearch index and write to an appsearch index. If you are already using logstash to index to your elasticsearch cluster than you could easily switch to appsearch output aswell.