Hello there,
Have you tried the Microsoft Store submission API and see if that helps in your case I think adding a third-party application might not help in the digital signature process.
After your app is published, you can publish an updated version by creating another submission in Partner Center. Creating a new submission lets you make and publish whatever changes are needed, whether you're uploading new packages or just changing details such as price or category.
This articlehttps://learn.microsoft.com/en-us/windows/apps/publish/publish-your-app/create-app-submission?pivots=store-installer-msix describes how to create an app submission in Partner Center.
Hope this resolves your Query !!
--If the reply is helpful, please Upvote and Accept it as an answer–