Yeah, what you were really looking for is remote types:
https://dgraph.io/docs/graphql/custom/directive/#how-dgraph-processes-custom-results
Instead of creating a new schema in the database, a remote type is a pseudo type just for the custom dql to return the GraphQl properly. This is not clear at all in the docs, and a rather advanced technique.
There should be @remote fields on real types, but that does not exists for the moment.
J