Not trying to be that kinda guy, but I'm pretty sure it's possible. You can configure both the game and any DLC to have a zip file in the root folder. Other indie games do this for artbooks, for example.
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I think there's a misunderstanding. I'm afraid Steam doesn't let developers release games with Android APKs secretly included when they don't support Android launches. They check for files that shouldn't be there and will force it to be changed before it can be allowed on their storefront. It's not like Itch.io where you can add pretty much any kind of file type you want.