I have configured a build for an Android project in Bitrise. Now I need to setup automatic deployment for beta testing in Fabric/Crashlytics, just don´t know how to add a step to accomplish that. Any ideas?
The same way you would on your Mac/PC.
You have to configure your project for Fabric/Crashlytics, following their official guides. At the end of the setup (current guide link: https://fabric.io/kits/android/crashlytics/install) you'll have Fabric/Crashlytics integrated into your Gradle project.
After the setup, to deploy to Fabric/Crashlytics, just run any of the Gradle tasks which invoke the Fabric/Crashlytics deploy.
On bitrise.io / with bitrise CLI you can run the Gradle task e.g. with the Gradle Runner step (or with a Script step).