If the API you want to call is well documented, then yes, and it’s pretty easy to do so. Just use Req to send HTTP(S) requests. You don’t even need to know what backend technology of the API you call, as long as it’s a JSON API over HTTP(S).

No, unless I am reading you wrong. Are there any programming languages that can freely embed each other? I mean OK, you can do so with Erlang and Elixir and there are other pairs as well like Java and Clojure (or any other JVM-living language) but to just get a collection of working HTTP endpoints and smack them inside an app made in another language is not supported out of the box pretty much anywhere.

You are better off just putting a reverse proxy like nginx or Caddy in front of both the Node.JS and Elixir app and have it decide which requests are served by which app.

…Or if you actually want to call the Node.JS app from Elixir, as in, do HTTP requests to it, then that’s a whole different story. Req will sort you out there, easily.