Great questions! With Shopify, it’s only possible to to export products via a CSV file. But as you touched on you need them in a xlxs file for what you’re trying to do. This will need to be converted externally outside of Shopify, once you have the CSV file. There is a good converter I found for this which seems like it might be possible to do for free. It’s called CloudConvert and you can see it here.
We have published an xlsx export format for Shopee on the Sheet Master app (https://apps.shopify.com/sheet-master) which is currently free. Happy to customize/fix this to best suit your needs. Please let me know at anto@comstack.co