Bro you just remembered me of the era I just created games in PowerPoint. The good old days! I would continue to do it for more time now but still, that's actually pretty nostalgic. If I get to find your game, I can assure you I'll play it and rate it! Bonus for the extra effort!
I think you could create a zip file / rar file, put your Powerpoint in and then submit it as a downloadable file. It might not get as many plays as a browser version may get, but I don't think there's a way to actually have a PowerPoint in itch.io. Maybe you could try and put the Powerpoint in a zip file? Idk. If you try that, make sure to create a backup of your game, just to be safe.
Hope this helps you out! Whenever you get to finish your game, feel free to ask me to play it / rate it.
:D