Is it possible to make an extension to change app icon

There’s an alternative solution:

Simply set a different icon in the Kodular app settings.
Build an .apk file.
Upload it to a drive.
Reset the default icon in your app.
Make sure your APK with a modified icon is downloaded from a URL.
After downloading, prompt the user to update the app, and therefore the icon.

It’s not the best solution, but it’s all I can offer.