MSIX appinstaller - how to include additional files

Those files need to be included in the MSIX package reference by the appinstaller, not in the appinstaller itself.

From what I know, Visual Studio does not have an option to include in the package files that are not referenced in your project.  You have two options:

1) Use the free makeappx.exe tool and replace the VS logic of building your package with a script you create. This tool is used in the background by VS and MSIX the packaging tool.
https://learn.microsoft.com/en-us/windows/msix/package/create-app-package-with-makeappx-tool

2) Use a third-party paid tool like Advanced Installer (the cheapest edition is enough) to configure the contents of the package. Tools like Advanced Installer give you a GUI where you can easily add all the files that need to be included in the package, using an explorer-like experience. After you can configure the project in Advanced Installer you can build the MSIX and the appinstaller. Here is a video showing how to do this, directly from its Visual Studio free extension.
https://www.youtube.com/watch?v=xmYW-YbEjD4

More powerful editing options are available in the Advanced Installer GUI, just use the option "Edit in Advanced Installer" available in the VS extension to continue editing the project.

Disclaimer: I work on the team building Advanced Installer.