I have a Samsung Galaxy Watch 3. It has Spotify with offline access as well. Really nice feature to have. I believe other Samsung watches have it as well.
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There are also some hacks for smartwatches that are not on the list to get offline Spotify to work. However, would suggest to use one that is officially supported.
As far as I know, that won't work. But you can use a third-party tool like AudBite Spotify Music Converter to download Spotify music and then transfer it to your smartwatch.