You won't be able to do that, because yes, that will be copyright infringement and theft.
You can, however, get those slowed-reverbed songs on your Local Files, which then allows you to make playlists of them on Spotify. You'll be the only one able to listen to these, as the songs in question are stored on your device.
See this article for more information about Local Files feature!
Hope this helps, have a lovely day 🙂
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