Welcome to the forum 🙂
As of now, REI3 can serve as REST server, so you would define an API in REI3 and then build a tool or script that reads from your restful API and calls the REI3 API. If you want to go that route, you can click-together an API in REI3 and the UI shows you examples of the REST calls you need to make (both for authentication and data exchange).
With the next release (3.4), which will be out very soon, REI3 can execute REST calls to retrieve or manipulate data from a REST server. It sounds like this is more what you had in mind. When REI3.4 drops, there will be REST templates included in the Builder that directly shows examples on how to call REST APIs from inside REI3.
