Federation with additional services without a graphql server

I’ve a Apollo Federation setup that work fantastically. My sticking point is I am tasked with folding in additional services/sources which for business reasons can not be altered - by that i mean adding a graphql server to each.

My current set up is a Node.js/Express that uses Rover to compile a super graph, and ApolloGateway ("@apollo/gateway": “^0.46.0”).

One solution is to create a mini-monolith graphql server that just serves these additional services. But I am hesitant to create another microservice that needs to be built and maintained. Any solutions?

I found this solution but is a few years old and uses methods that are marked as deprecated (e.g. serviceList)