Can I upgrade my Supporter pack through Steam?

This will depend on if this is selected as a "new purchase" or as an "upgrade" to your existing pack. For the former, what you've asked will be true, just as how players are able to purchase multiples of the base "Early Access Supporter Packs" in order to receive more Early Access keys. As an upgrade to an existing pack you have purchased, it will not be true, and you will simply receive everything else from the tiers higher to the pack you already own.

If you have purchased the "Early Access Supporter Pack" from Steam, and are looking to purchase a higher tier pack on Steam, this may result in an "Upgrade" instead.

I hope this clears things up for you!