It would be great to be able to add a survey from an existing feature service, assuming it's properly set up. This would allow us to use our Server through AGOL...
Have you read the Blog Entry from 2016-jul-29?
Survey123 for ArcGIS
It works very good!
Submission URLs should be what you're looking for. You still need to build the survey manually, but with submission URLs you can feed it into an existing feature service.
Use Survey123 with existing feature services—Survey123 for ArcGIS | ArcGIS
Retrieving data ...