You can just do it. Itchio doesn't take a part of in-app purchases.
Viewing post in IAP in itchio
Haha funny you mention it, I actually have a thread suggesting they update the Reward system into the API so it can be used for in-game content XD
https://itch.io/t/75228/suggestion-rewards-api
If you agree it'd be a cool system I'm sure them seeing some comments in the thread would make it more known to Amos and Leafo ^_^