I'm not sure if there is a way to buy DLC on itch retroactively. I've checked around and haven't found a way so far. But the game will release on Steam shortly and those who bought the game here can claim a Steam key. On Steam you can buy DLC on it's own because the game and DLC are treated as separate purchases.
If you don't want to wait until around March 10th or you simply don't like Steam, let me know and I'll try to come up with another possibility :].