OK, maybe the lambda stuff is tripping you up. If you have a graph with a schema active, then writing a program in pretty much any language you choose that makes a REST API call to some other service can then easily add that returned data to your graph.
Maybe check out the clients section of the docs (https://dgraph.io/docs/clients/overview/). There are a number of Dgraph supported/recommended clients listed there. Note these are primary for DQL, not GraphQL. If you’re looking for GraphQL clients, then depending on the language you choose to write your REST API calling program, you’ll be able to find many GraphQL clients in open source.