I kinda had the impression that it's been an intentional decision not to add this, because very few developers use microtransactions/lootboxes properly, and even standard DLCs can be really cash-grabby, not-adding-a-lot-of-value-y, so the overall quality of games posted to Itchio will be higher if people can't monetize their games using them... usually the logic between using micro-monetization models is to have a ton of really small trickles of income, and it pairs really well with a "upload tons of similar, small games using the same system" model.
Also, apart from making DLCs as separate games, there's already a system where you can have files that are only available if the customer pays more than the minimum required price for your game. This would let you have a "complete edition" with added DLCs as a separate download. Won't really work if you want to have multiple separate DLCs (e.g. "buy every character separately") but it could work if you plan to make an "expansion pack" later that contains a bunch of random extra content.