Hi @rene,
It is currently not possible to build custom apps and use them in Automation in Freshdesk and Workflow Automator in Freshservice.
In Freshdesk, the Slack app is an internal feature exposed as an app for optional onboarding for customers who wish to use it. So, we do not have the platform integrated with the Automation before and it will be a brand new feature when we have to support custom apps exposing custom actions.
In Freshservice, the platform works only with certain apps that are published as Marketplace apps currently. It is not exposed for anyone to build apps that are compatible with Workflow Automator yet. However, it is in the roadmap to release it soon so that anyone can build apps with custom actions that can be used in Workflow Automator.
I will pass your request as feedback to the product team.
For now, you might have to use the webhook action to hit any external system APIs. If you would need any computation through the apps, you would have to use any of the available product events for your application.