Hello! In an open issue, Additional action types for alerting · Issue #45023 · elastic/kibana · GitHub, this looks like it's on our roadmap.
I haven't been able to find exactly what you're looking for (case creation from Elastic using Cortex XSOAR) but I wanted to share what I have found.
I found a Palo Alto Networks presentation where they integrated XSOAR with Elastic.
Presentation: Upgrade Your SOC with Cortex XSOAR & Elastic SIEM | Elastic slides: Upgrade Your SOC with Cortex XSOAR & Elastic SIEM
It looks like they have some more information on their site that might be helpful:
https://docs.paloaltonetworks.com/cortex/cortex-xsoar/6-0/cortex-xsoar-admin/installation/install-cortex-xsoar-with-elasticsearch.html
https://docs.paloaltonetworks.com/cortex/cortex-xsoar/6-0/cortex-xsoar-admin/manage-data/migrate-indicators-to-elasticsearch/indicator-migration-overview.html
Thanks!