It’s surprisingly easy once you give it a try, although you might need to dedicate an hour or two to working out the fiddly parts.
If you (or anyone else) is interested, all you’d really need to do is open the medieval items mod’s folder, find the text file that has the definition for a similar helmet, copy/paste the similar helmet, adjust the stats/name to your liking, find the text file that has the itemgroups that have the similar helmet, and paste the item ID for your new helmet into the same itemgroups.