Looks like it's really easy. Here's a relevant quote from Itchio's helpful article:

Late submissions happen, for whatever reason an entry isn’t able to make it in time for the deadline. Luckily itch.io’s jam feature now lets owners of a jam handle this with ease. From the submissions tab you can find the late submission form. Just paste the URL of the game you want to have late submit access, you’ll then be given a URL to share with the game’s owner. They just need to visit that URL to submit their game, even if the submit deadline is over!

So, all I'll need is the URL for your game. If you're not comfortable posting that directly here, you can send it to me personally at glenarvon @ gmail.com and I'll do the rest.