Can't add Windows & Xbox program to existing Partner Center publisher

We have an existing, verified Partner Center account (publisher "CMiC Global", enrolled in Commercial Marketplace and the AI Cloud Partner Program) and need to publish a Windows desktop app (MSIX) to the Microsoft Store under that same publisher.

What we've tried:

Account settings > Identifiers > Publisher > Add program only offers Microsoft 365 and Copilot. Windows isn't listed.

Account settings > Programs > Other programs > Windows > Get started redirects to storedeveloper.microsoft.com, which starts a brand-new company sign-up. After D-U-N-S verification, our publisher name "CMiC Global" returns "Publisher name is not available" because our existing account already holds it.

Question: What is the supported way to add the Windows & Xbox program to an existing publisher without creating a separate developer account? Has anyone else gone through this?

Support cases 2610200040009261 (Partner Center, opened Oct 20) and 2610260040007082 (Dev Center, opened Oct 26) haven't had a response yet, and this is blocking a product release. Any guidance, or help routing the cases to the Store onboarding team, would be appreciated.

Also posted on Microsoft Q&A: https://learn.microsoft.com/en-us/answers/questions/6051001/