You can have multiple versions of a game at different price points, so you could have base game with all DLC disabled + base game with all DLC installed available as different options. (For instance either including the expansion files in the full-package version, or compiling two different versions of the executable with DLC checks hardcoded to either "no" or "yes") There's no itchio-intrinsic way to make it more granular than that at the moment, though. (You can have any number of tiers but it doesn't really make sense for DLC that don't have ordering dependencies, like character packs)