@Jurre Lagerweij
Thank you for your post!
When it comes to creating duplicates of your wildcard certificate with additional Subject Alternative Names (SANs), this currently isn't support within the Azure Key Vault. However, if you'd like this to be a feature, I'd recommend leveraging our User Voice forum and creating a feature request, so our engineering team can look into implementing this. I've also created an internal feature request, so our engineering team is aware of this as well.
Links:
Integrating Key Vault with Integrated Certificate Authorities
Integrate Key Vault with Integrated Certificate Authorities FAQ
Key Vault REST API - Certificate operations
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
----------
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.