Hi! I've not figured out the best way yet as itch doesn't really have a good mechanism for selling DLC (I guess I could just make a page with a zip file to download) - although if you redeem the Steam key which came with the itch version and then get the DLC, you can copy the DLC folder to the itch version directory, and the game will find it.
Of course, I understand that this isn't ideal if you don't want to use Steam at all, but FWIW the Steam download also includes the itch/non-DRM version in the install directory. It's usually more up to date than the itch download too (and also has a beta version), as it's easier to push incremental updates to Steam than it is to itch.