In kubernetes, using an nginx ingress controller what is recommended way of adding back end services?
Ingress controllers allow you to not manually wire stuff up, that's the whole point.

If you'd rather do it manually You could have an nginx array (either inside Kubernetes our outside of it) and just point to the ip/port of the services that expose your microservices. If you use an nginx array external to Kubernetes you'd have to use the externalIP service type. If you run the Nginx array on Kubernetes you could store your "manual proxy config" in a configmap so that all your nginx proxies get the same config.

I would definetely advise to not do this tho. Ingress controllers are built to simplify traffic routing, and as your complexity grows you will definetely want to take advantage of them. A good labeling scheme is absolutely paramount in Kubernetes, and that goes for ingresses aswell.