If you are talking about self-signed certificates used to enforce the policy that only "signed" runbooks should be run on hybrid runbook workers, as mentioned here - Work with signed runbooks on a Windows Hybrid Runbook Worker. These certificates are available in HybridRunbook worker itself (you can choose to upload it to Azure Automation's certificate also)
For this, you can export it from runbook worker server by following the steps below:
1. Start --> Run --> certlm.msc
2. "Trusted Root Certification Authorities" --> "Certificates". Right click on the certificate --> All Tasks --> Export.
I have also added a reply to your question for RunAs Account Certificate exportation here - https://learn.microsoft.com/en-us/answers/questions/1101911/index.html
Please let us know if you have any questions. If I have misunderstood the question, please provide additional details to help me understand it better.