At the moment we do not support adding a new layer to an existing feature service. We do have plans to investigate adding this ability and will update this thread once complete.

Any updates on this feature?

Hi @demeird,

Unfortunately there doesn't seem to plans on this one at this time. If you're interested in this functionality, you can upvote this existing idea here to help inform our development team of your interest. It would also help to share your use case and how you would like to see this added ideally. Sorry for the inconvenience!